Email is the medium of business. It's how requests, deals, hires, are started and made. 

Most businesses live in their inbox, whether they like it or not. And that inbox is likely an Outlook inbox — over 3.7 million companies use Microsoft Outlook for email management. 

The are two main reasons for that:

1. Outlook is the default email client for Microsoft 365 which is a suite that many businesses use—Excel, Word, and Teams. 
2. They offer enterprise-level security, compliance, and control—GDPR, HIPAA, encryption, granular user permissions—you name it.

However, like Word or Excel, Outlook was made mostly for enterprise solo use. It wasn't made for collaboration, even as the world of business and email moved towards needing more and more collaboration. 

In 2025, several tools meet the security and control standards of Outlook while offering far more powerful inbox collaboration and coordination features suited for modern businesses.

We'll cover what to look for in an Outlook email client, introduce the six most popular third-party options, and break down their key differences.

Here are the key features we looked at for Outlook email clients: 

• User interface and experience
• Security and privacy
• Customization options
• Collaboration features
• Productivity features
• AI specific features

All options have desktop and mobile email apps and support IMAP, MAPI, and POP3.

Plus, we'll cover a range of price points for the best Outlook alternatives—including ones that are free email clients.

Missive

Missive is a collaborative inbox for teams that run on email. This means it is designed with collaboration as a priority, featuring contextual in-email chat using @mentions—eliminating the need for forwarding.

You can assign or watch emails, and every action is logged—giving you visibility into emails and tracking who did what and when.

On top of that, Missive supports all email providers (Outlook, Gmail, Yahoo, Apple Mail, etc) and you can have multiple accounts (personal and business).  

Under the hood, Missive has a powerful automation engine, allowing you to do things like:

• Route emails from a specific sender to certain people/teams.
• Create a service level agreement (SLA) with notifications to ensure a high standard of service.
• Have AI read and understand the contents of an email to see if there's a deadline, label as urgent if there is, and assign it to a senior staff to reply.  
• Have AI read and understand the contents of an email to see if it's spam, and automatically trash. 

From a security perspective, Missive meets the same gold standard as Outlook. They have an SOC 2 Type II report, encryption of data at rest and in transit, and they are GDPR compliant. 

For pricing, Missive plans start at $14/user/month on an annual plan.

One thing to note, if you use folders in Outlook, they are called labels in Missive.

In the same way that some teams prefer Google Docs to Word because of their collaboration functionality (commenting, multi-player drafting, etc) — you may prefer Missive as your email app to Outlook, if you find yourself hitting reply all and forward all the time.

Thunderbird

Thunderbird stands out as the only open-source email client.

It's a community-driven, free email client, that has been around for nearly two decades. With a thriving online community and an ecosystem of 1200+ add-ons (including AI-powered ones to help you draft replies), it's considered one of the best email apps for those prioritizing a free and open sourced solution. 

If you're looking for an email client that has more collaboration functionality, Thunderbird's collaboration features come mostly from its third-party add-ons—things like mail merging and adding notes/comments to emails. Which makes collaboration possible, but likely a little unreliable given the nature of third-party connections failing from time to time. 

From an organization perspective, Thunderbird calls their version of "folders", tags. Functionally, they are the same. 

Thunderbird is a very privacy forward email app with built-in filters for phishing/spam and remote image blocking.

Though, it doesn't have the same compliance certifications (i.e. SOC or ISO) due to it's free and open-source nature. 

Mailbird

Mailbird is for those of you who have way too many email accounts. It's known for it's unified inbox, where you're able to flow multiple accounts into the same consolidated inbox view.

Mailbird doesn't offer any features related to collaboration or coordination. It's more of a productivity improvement for Outlook power users who would like to integrate a few popular apps into their email workflow and see all emails in one place.

From an AI perspective, Mailbird offers simple AI drafting through ChatGPT. 

Of all the Outlook alternatives on this list, Mailbird has the most similar user experience to Outlook—for example, their naming conversions are the same (folders are folders, and not labels or tags). 

For security and compliance, Mailbird is only GDPR compliant and does not have any external audits or certifications. 

For pricing, Mailbird has a free version as well as a premium version that's $4.99/user/month. There is also a pay once option to buy the product outright at $49.50 (standard) or $99.75 (premium). 

If you manage multiple Outlook accounts and need a unified inbox for all your emails, Mailbird might be the perfect solution.

eM Client

eM Client is a very similar email client to Mailbird. Most of their features are productivity focused for individuals—shortcuts, watch/snooze, configurable layout. 

The most unique and powerful feature for eM Client is their search. Not only does the search cover all messages in your inbox, it can also search within certain types of attached files—think PDFs, Word docs, etc. 

On the collaboration front, they don't have much beyond the ability to share folders (aka. labels), calendars, and accounts. 

Like Mailbird, eM Client offers basic AI drafting to assist with typos and tone in your replies.

On security and compliance, eM Client is GDPR compliant (though possibly outdated with 2018 references) and does not have any external audits or certifications. 

For pricing, eM Client has a sharp distinction between personal and business plans. There is a free plan for non-commercial use. The paid plans can be an annual subscription or a one-time payment. 

The personal plan (without AI features) is $39.95/year or $49.95 as a one-time payment.

The business plan (with AI features) is $49.95/year or a one-time payment of $188.95.  

Both one-time payment options do not include future feature updates. You can purchase lifetime upgrades separately at $90 per license. 

If you're looking for a slightly more productive version of Outlook and you want a free email app because you're not using it for commercial purposes—then eM Client might be a good option.

Apple Mail

If you're a Mac user and you really don't want to download another email client. Does the out-of-the-inbox (get it?) mail app from Apple work well for Outlook? 

Well, compared to Thunderbird, Mailbird, and eM Client—Apple Mail isn't going to give you any increased functionality. 

If you use Apple Mail as your Outlook email client, you won't have the integrated calendar or task management, and you'll have to remember that folders are "labels" in Apple Mail. 

The good news is that Apple Mail can support multiple accounts from multiple providers (via IMAP and SMTP standards), so if you have a Gmail account and an Outlook account that you would like to unify into one very well designed, simple inbox—Apple Mail can do that.

If you want a free email client with a cleaner design than Outlook and don’t require advanced features, Apple Mail might be your best option.

Superhuman

When Superhuman first came out, it was solely focused on Gmail and Google email users. As of May 2022, they also support Outlook users. 

From a user interface perspective, Superhuman is the most distinct of this list. It looks nothing like an Outlook inbox, so if familiarity is a requirement, this might not be a good fit. 

Superhuman offers several AI-powered features, the most notable being its ability to answer questions about your inbox.

Instead of traditional search (even as powerful as eM Client's), you can ask your inbox direct questions. Instead of needing to remember a file's name to look for a specific piece of information, you could say: What was the price that John from ACME quoted me?

On the collaboration front, Superhuman offers the ability to @mention your colleagues through Team Comments.

From a security and compliance perspective, they are compliant with SOC 2 Type II, ISO 27001, CCPA, and GDPR. 

For pricing, Superhuman is on the higher end of these email clients, starting at $25/user/month on an annual plan.

Use Cases and Scenarios

To summarize our options for the best email client for Outlook users, we sorted them into two categories: 

Individuals (You just want a better client for yourself)

• Thunderbird—you want open-source and lots of plug-in options
• Mailbird—you want Outlook, but a unified inbox
• eM Client—you want robust search
• Apple Mail—you want a better design, with less functionality
• Superhuman—you want AI and shortcuts galore

Teams (You want an email client for coordination and collaboration within a team)

• Superhuman—you just want to be able to @mention instead of forward
• Missive—you want shared inboxes, internal @mentions, shared labels, canned responses, watch/assign... basically the best email client that's collaboration-first

We hope this has been a helpful overview of the types of email clients that are out there for Outlook users. If you're interested in Missive, continue on and we'll get into some tactical information. 

Recent heavy Outlook users who made the switch to Missive:

Stephanie at Lighting Dynamics, manages 100+ email quotes a day. Her team used to use Outlook for email management:

With traditional Outlook forwarding, once an email was out of the shared inbox, there was no visibility. We never knew if it had been handled. It was chaotic.

And now, with Missive: "Missive checked all our boxes. It was a huge relief to see we could maintain the shared inbox model—without building custom software from scratch."

Or Kason, from i-SOLIDS, who grew his sales team beyond himself: 

We got to a point where we weren't providing the same level of communication, response, and service that allowed us to get to this point. We were relying on Outlook email and it was like 'are you responding to that or am I?'

And after a month with Missive, Kason recommends: "Don't think about just choosing a tool for today but this tool needs to work for scale too—that's a major decision factor."

How Missive integrates with Outlook

1. Create a free Missive account 
2. Select your email provider (either Microsoft 365 or Outlook most likely)

1. Determine how much history you would like to sync (only paid accounts have unlimited history)
   

1. Repeat for as many different email accounts (shared or personal) as you need. 
2. You have the option to sync contacts and calendar as well. 

Get a detailed walk through of how to configure Outlook to Missive, including terminology differences to get you acclimated to your new inbox.

So what's the best email client for Outlook users? 

Well like most things, it depends. If you're a team who lives in their inbox day and day out, and you're looking a collaboration-first inbox—we hope you'll give Missive a try. 

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We've all been there. You open your inbox on Monday morning and face an avalanche of messages. Some need immediate attention. Others could wait. Many should be handled by different team members. And a surprising number don't need any response at all. While Missive's rule engine has always given you the flexibility to automate your workflow exactly how you want it, today we're taking that customization power to a whole new level.

What if your inbox could sort itself? What if it could understand what each email is about and take the right action automatically—all while you maintain complete control over how it behaves? What if you could define exactly how your emails are processed, based not just on who sent them, but on what they actually contain?

Today, we're launching AI Rules in Missive - a simple way to bring the intelligence of AI to your email workflow without the complexity. It's the same flexible rule engine you love, now with the power to understand email content the way you do.

What are AI rules?

AI Rules are an extension of Missive's existing rules engine. If you've used rules before, you know they're powerful for automating repetitive tasks based on simple conditions like sender address or subject line keywords.

Now, we're adding the ability to use AI to understand what an email is actually about.

Here's how it works:

1. You create a rule with an "AI Prompt" condition
2. You write a simple instruction for the AI
3. The AI reads the email and gives an answer
4. Your rule takes action based on that answer

No training data. No complicated setup. Just plain language instructions that the AI follows.

Real examples, real time saved

Let's look at some practical ways teams are already using AI Rules during our beta:

- A ⛑️ customer service team set up a rule that using this prompt:‍

‍Is this customer angry or upset? Respond with ONLY "YES" or "NO".‍

If the AI says YES, the email gets flagged as high priority and assigned to a senior agent.

- A 📈 sales team created a rule with this prompt:

‍Is this a qualified sales lead or just a general inquiry? Respond with ONLY ONE of these exact words: "sales lead" or "general inquiry".

Leads go straight to the sales pipeline, while general questions route to the support team. Their sales reps now spend more time selling and less time triaging emails.

- A ⚖️ legal firm uses AI to detect if an email contains a deadline or time-sensitive request.‍

Does this email contain a deadline, due date, or time-sensitive request? Respond with ONLY "YES" or "NO".‍

If it does, it gets tagged "Urgent" and triggers a notification and creates tasks. They haven't missed a filing deadline since.

The best part? These teams didn't need to become AI experts. They just wrote simple instructions in plain English.

One prompt, multiple rules

One of the most powerful features of AI Rules is the ability to use the same prompt across multiple rules. This lets you create sophisticated email triage systems without duplicating your AI analysis costs.

For example, you could recreate Gmail's smart categories with more flexibility and control.

First, create a prompt that categorizes emails:‍

Analyze this email and respond with EXACTLY ONE of these categories:

"SOCIAL" - for messages from social networks, dating sites, etc.

"PROMOTIONS" - for marketing emails, offers, discounts, newsletters

"UPDATES" - for notifications, confirmations, receipts, statements

Then create separate rules, all using this exact same prompt but with different matching conditions:

The beauty of this approach is that the AI only analyzes each email once, even though you have six different rules. The result is cached and reused across all rules, making this both efficient and cost-effective.

And unlike Gmail's fixed categories, you have complete control over:

• What categories to use
• What actions to take for each category
• How to prioritize different types of messages
• When to apply these rules (you could add additional conditions)

This is just one example of how you can use AI Rules to create a customized workflow that fits exactly how you and your team want to work.

Four ways to use AI in your rules

We've built four powerful AI capabilities into Missive:

1. AI Conditions

As shown above, use AI to analyze email content and make decisions. The AI can detect sentiment, identify request types, or extract specific information that would be difficult to capture with traditional keyword rules.

For example: "Is this customer angry?" or "Does this email contain a deadline?"

2. AI-Generated Posts

Have the AI create a helpful note about an email. The AI can summarize long threads, extract key points, translate emails or provide context for your team.

e.g.

Translate the email to English.

3. AI-Generated Tasks

Let the AI identify action items in emails and automatically create tasks. No more manually creating to-dos from your messages. For example:

‍Extract any tasks or action items from this email and create a task for each one.

4. AI-Generated Drafts

Automatically create response drafts for common inquiries. The AI can craft a personalized reply based on the email content, which you can review and send with a click:‍

Create a helpful response to this customer inquiry about our pricing plans.

The above draft example could be paired with an AI condition that makes sure the email is about billing!

Simple to set up, powerful in practice

We've worked hard to make AI Rules approachable. You don't need to be a prompt engineer or AI expert to get value from day one. The system uses gpt-4o-mini, which offers an excellent balance of speed, cost-effectiveness, and quality for email processing.

Your data, your control

We understand that email contains sensitive information. That's why:

• You connect your own OpenAI account
• You control exactly which emails get processed by AI
• Email content is only sent to OpenAI at the moment of rule processing
• No data is stored by OpenAI beyond what's needed for the immediate request

The inbox you've always wanted

For years, we've been building tools to help teams manage email more efficiently. Rules have always been at the heart of that mission - letting you automate the repetitive stuff so you can focus on the work that matters.

AI Rules take that automation to a new level. Now your inbox doesn't just sort emails based on simple patterns - it understands what they're about and what needs to happen next.

This isn't about replacing human judgment. It's about removing the tedious parts of email management so your team can focus on what humans do best: building relationships, solving complex problems, and delivering value to your customers.

AI Rules are available today for all Missive users on the Productive plan and above. Give them a try, and let us know what you think.

Your inbox will thank you.

Frequently Asked Questions

Want to learn more about AI Rules? Check out these helpful resources:

• Which OpenAI model is used for the AI rules?
• How do I set up AI rules in Missive?
• What plan do I need to use AI Rules?
• How do AI conditions work?
• Can I reuse the same AI prompt across multiple rules?
• How do I write deterministic prompts for reliable rule matching?
• How do I debug AI condition results?
• What's the difference between the three AI actions?
• How many tasks can be created with the 'Create AI Tasks' action?
• Can I customize the AI-generated tasks?
• What are the content limits for AI analysis?
• Will using AI rules slow down my email processing?
• What happens if the OpenAI API is unavailable or returns an error?
• How can I optimize my AI rule usage for cost efficiency?
• Can I combine AI conditions with regular conditions?
• What are some example use cases for AI Rules?
• What are some example use cases for AI Rules?
• How is my data handled with AI Rules?
• Do I need to be an AI expert to use AI Rules
• What is the difference between standard rules and AI rules?