Email Management for the Modern Inbox
In the industry dominated by Microsoft Outlook, many users end up exploring alternatives for email management, and both eM Client and Mozilla Thunderbird frequently populate lists and online discussions of the best Outlook replacements.
Thunderbird started as a Mozilla Foundation project in 2004. When the project’s future became uncertain in 2017, Thunderbird was revitalized as community‑driven software, and today it’s owned by MZLA Technologies Corporation, a Mozilla Foundation subsidiary.
eM Client was founded in 2007 in the Czech Republic and has experienced steady growth, expanding from Europe to a global user base over the years. In October 2024, eM Client announced the acquisition of Postbox, and this expansion solidified eM Client's presence as a dedicated email client.
How do these now‑established players in the field of email clients compare today?
Note: As open‑source software, Thunderbird allows tech‑savvy users to benefit from community‑made extensions and contributions, but for this comparison, we’ll focus on Thunderbird features directly supported in the standard download from Thunderbird.net.
The Desktop App: Beyond Email
First things first: Can you install the software? eM Client’s desktop version is available on Windows and macOS, while Thunderbird’s download options also include Linux. As for mobile, you can install eM Client whether you use Android or iOS, while Thunderbird offers an Android app. More on mobile capabilities later!
Launch either client and get started by adding your first email account. eM Client and Thunderbird both work with the standard email protocols IMAP, SMTP, and POP3, or Microsoft's proprietary protocol, Exchange Web Services (EWS), via a smooth, automatic setup for most known email providers. Both can also connect to Microsoft 365 accounts through the Microsoft Graph API, which Microsoft recommends in place of EWS as it phases EWS out.
Looking for a secure email client? Both deliver, with full support for PGP, S/MIME, and SSL/TLS encryption. Additionally, note that installing add‑ons in Thunderbird means users should verify their sources, while eM Client’s integrated approach ensures secure releases.
All that said, both clients strive to provide a comprehensive experience beyond “just” email and stand as an all‑in‑one productivity toolset. Let’s explore how well they deliver on that promise.
-
Chat
To chat in eM Client, you can add your accounts from XMPP, Slack, Teams for Business or Enterprise, Rocket.Chat, and IceWarp Teamchat. Thunderbird supports XMPP, IRC, Matrix, and Odnoklassniki.
eM Client's support for Slack and Microsoft Teams, including channels, makes it particularly valuable for workplace environments where these major platforms are already in use.
-
Calendar and Contacts
Both clients support local and server‑side calendars and contacts. You can connect your calendar along with your email accounts or add any CalDAV or CardDAV accounts separately later. Google Calendar uses its own protocol, similarly supported in both clients.
Thunderbird also handles contacts via the LDAP protocol. When it connects through EWS or the Microsoft Graph API, it only syncs email so far. eM Client syncs email, calendars, contacts and tasks through both.
-
Tasks
If your server supports tasks, both clients have you covered. If not, you can still manage tasks locally in either app.
-
Notes
Notes are what gives eM Client a competitive edge in the feature range. Write them locally or add a compatible CalDAV account to handle notes within the eM Client app with all your other tools.
Make Your Client Your Own: Customization
eM Client is localized for 25 languages, and Thunderbird boasts a total of 65 – regional variants of languages like English included in the count.
Both apps offer dark, light, and auto themes. eM Client offers 12 additional themes in‑app, and you also have the option to really dive in and create your own theme with extensive color customization, share your theme, or download ones created by the community. Another strong point is the ready font customization options for most of the interface. Thunderbird has many more themes available in the browsable add‑ons library.
Free Forever vs Feature‑Rich
Does any of this set your wallet back? It depends.
You get access to the full scope of eM Client's features for the 30‑day trial duration. After that, you can continue with the free version or explore paid options. For personal use, the annual subscription costs around $40. One‑time payment licenses start at approximately $60 for the current major version, or around $150 for a license that includes all future upgrades. If you’d rather decide whether each future update is worth your money after its release, a single upgrade costs around $35. Each personal license covers up to 3 desktop installations.
eM Client’s Business subscription costs around $50 per device, while the one‑time payment option costs around $80, or around $180 with lifetime upgrades. Business licenses include access to the License Manager, which allows admins to centrally configure settings across all company installations.
On top of the community support forum, eM Client prioritizes the user experience with its VIP Support, included for one year with each purchase, with yearly extensions available for around $10.
Thunderbird operates as a fully free, donation‑supported model, with an optional paid service, Thunderbird Pro. It bundles Thunderbird's own email provider, Thundermail, with the scheduling tool Appointment and encrypted file sharing (Send). As of November 2026, it's in beta and available in waves to people on the waitlist, with an Early Bird plan at $6 per month, billed annually.
AI is available in eM Client Desktop as a $2 monthly add‑on for all users or included at no extra cost for annual subscribers. By default, the AI features use OpenAI, and text used for them is sent only to the AI provider and isn’t stored or logged by eM Client. With any paid license, you can also bring your own key (BYOK) to use the AI service of your choice without the add‑on. It offers various tools to improve email writing, from light touches like proofreading for grammar and word choice to composing entire replies in your own writing style. eM Client 11 added an AI Assistant in the sidebar, AI summaries of emails and conversations, one‑click quick reply buttons, natural‑language search, AI‑generated rules and creating tasks and events from messages. MCP support, which lets AI agents connect to your mailbox, doesn’t require the AI add-on – it’s available with any paid license.
Note: The listed prices are accurate as of October 2025. For current prices, visit eM Client’s pricing page.
In eM Client Mobile, the AI feature is currently included for free. Speaking of which...
From Desktop to Mobile
2024 saw both eM Client and Thunderbird come out with mobile apps, both completely free. As mentioned earlier, eM Client develops a dedicated app for Android and iOS, making it a convenient choice, particularly for users invested in the Apple ecosystem, and Thunderbird Mobile, which evolved from the open-source K‑9 Mail, is available on Android.
Both clients offer a quick method of exporting settings from desktop to mobile via a QR code, though the extent of transferable settings differs between the two clients. Thunderbird’s mobile app focuses primarily on core email functionality. If you prefer an app solely focused on email management, Thunderbird delivers on that front. If you want access to the same functionality you’re used to on desktop, eM Client’s release in October 2025 brought the calendar, contacts, tasks, and notes features to the mobile app as well.
In the Google Play Store, both apps maintain solid ratings of 4.0 stars, based on around 1,000 reviews for eM Client and 5,000 reviews for Thunderbird at the time of writing this article.
eM Client has amassed over 100,000 downloads in Google Play while Thunderbird’s downloads stand at over one million and rising.
Is It a Match?
eM Client provides an easy-to-use solution for anyone looking to migrate from Outlook or Postbox. It comes with all the essential features and more, and you can make your experience as simple or as complex as you prefer. The one-time payment option offers good long-term value.
eM Client is also well-suited for professional needs, given the availability across operating systems, Teams and Slack support, and the dedicated License Manager.
Linux fans and those who enjoy building on top of the base application with a library of add-ons and plugins will appreciate Thunderbird. The open-source nature and active community development mean you can customize nearly every aspect of the email client to fit your workflow. For those prioritizing budget, Thunderbird delivers a complete experience at no cost.
Ultimately, both eM Client and Thunderbird are powerful email clients that prioritize user experience. The right choice depends on whether you value integrated features and professional support, or open-source flexibility and community-driven development.