People leave Outlook for all sorts of reasons — frustration with the new interface, wanting to cut Microsoft 365 costs, or simply needing a different workflow. The good news is there's no shortage of capable desktop clients. eM Client, Thunderbird, Mailbird, and Spark each have their own strengths, but they differ a lot in the factors that determine a migration: Exchange support, calendar and task depth, mobile apps, and how much you get without paying.
Here's how all four actually compare — and why eM Client tends to be the one people settle on.
The alternatives, compared
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eM Client (our pick)
eM Client supports Exchange and Microsoft 365 accounts natively, matches Outlook's depth on calendar and task management, and runs on Windows, Mac, iOS, and Android.
The Free license covers two accounts for personal use, and a paid license lifts that limit and unlocks commercial use. You can buy it as a subscription or as a one-time purchase (with an optional Lifetime Upgrades add-on for all future versions), so you're not locked into an ongoing Microsoft 365 subscription just to get full functionality.
The iOS and Android apps are free to use with no license at all.
It can also import your existing mail, contacts, and calendar directly from Outlook — either during setup or later from the settings — which keeps migration simple.
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Thunderbird
Thunderbird is free, open source, and runs on Windows, Mac, and Linux, plus a stable Android app (iOS is still in development, with the team building it from scratch through 2026).
It added native Exchange support in late 2025 via EWS, but that currently covers email only — calendar and address book are on the roadmap, and Mozilla has said that work is paused until Microsoft Graph support lands, which is now the team's priority.
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Mailbird
Mailbird covers Windows and Mac. It has useful built-in integrations for apps like Slack, Microsoft Teams and Google Calendar, and it connects to Exchange and Microsoft 365 using the Exchange protocol, though that's a Premium feature.
You can add a Microsoft account over IMAP instead, but that only brings your email across, not your calendar or contacts. The free tier is also limited to a single account, so it works as a trial rather than a unified inbox.
There's still no mobile app; it's listed as a roadmap item, not something available today.
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Spark
Spark runs on Mac, Windows, iOS and Android and connects to Gmail, Outlook, Exchange, iCloud and other IMAP or EWS accounts (it doesn't support POP3). The app is free to download and use, with a Premium subscription for individuals and teams.
One structural difference worth knowing: your added accounts and settings are tied to a Spark Account that syncs them across your devices, rather than staying on your own machine as they would in a local desktop client.
A note on Exchange and the EWS deadline
This one applies to any client you're considering, not just a specific app. Most third-party email clients reach Exchange servers through Exchange Web Services (EWS) protocol, and Microsoft is retiring EWS for Microsoft 365 accounts on October 1, 2026, in favor of the Microsoft Graph API. On-premises Exchange Server isn't affected. What differs between clients is how ready each one is for the switch, so it's worth asking before you migrate.
eM Client is already there: Graph support arrives in eM Client 11, which is available to try in beta now — no license needed for testing. For the full picture of what the retirement means and how to prepare your account, see eM Client's post on the EWS retirement.
Thunderbird has made Graph its next development priority, which is why its Exchange calendar and address book work is on hold. Mailbird and Spark both connect over EWS today, and neither has published any plans for MS Graph support so far.
What to consider before switching
Compatibility with Exchange/Microsoft 365 accounts — eM Client and Spark support this natively; Mailbird does too, but only on Premium; Thunderbird's Exchange support is email-only for now.
Calendar and task management depth, since this is where Outlook is strongest — eM Client is the closest match; Mailbird and Spark lean lighter here.
Mobile access — eM Client and Spark have native mobile apps (eM Client's are free and need no license); Mailbird doesn't, and Thunderbird's mobile app is Android-only so far.
Migration effort — eM Client, Thunderbird and Mailbird all have an import tool; Spark doesn't.
Long-term Exchange support — check where the client stands on Microsoft Graph ahead of the October 2026 EWS cut-off; eM Client is adding full Graph support in version 11.
FAQ
Is there a free alternative to Outlook that still supports Exchange?
Yes — eM Client and Spark both support Exchange and are free to start with. Mailbird also connects via Exchange, but it's a Premium feature rather than part of its free tier.
Will I lose my calendar and contacts if I switch?
Not with eM Client. Your Outlook calendar and contacts come across with the import, and from then on, they stay in sync with your account. It's worth checking before you commit to one of the others, though: Thunderbird can't sync calendar or contacts from an Exchange account yet, and Mailbird needs the Exchange protocol, a Premium feature, to sync a Microsoft calendar at all.
What's the best alternative specifically for Windows?
eM Client, since it matches Outlook's feature depth on Windows while also running natively on Mac and mobile if you need that later.
What if my team needs to collaborate on email?
Spark is the only one of these four with collaboration built into the app, including shared drafts, shared threads for discussing emails, and delegation. With eM Client, Thunderbird or Mailbird, team collaboration comes from the mail server instead — shared mailboxes, shared calendars and delegated access set up in Microsoft 365 or Google Workspace.
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