If you're the kind of person who cares about open source software, you've probably already noticed that the email client space is dominated by products that want your credentials on their servers and aren't particularly transparent about what they do with them.
This matters more for email than for almost any other category of software. Your inbox is not just a list of messages. It's a complete record of your professional relationships, your business discussions, your financial transactions, and your personal life. Handing that to a company whose privacy policy you haven't read carefully is a meaningful decision, even if it doesn't feel like one.
This post looks honestly at what open source email options exist for someone managing multiple accounts, what they actually give you, and where the current state of things falls short.
Before going through the options, it's worth being clear about terminology, because the space is messier than the label suggests.
A truly open source email client is one where the application code is published, auditable, and modifiable. You can read what it does, verify that it doesn't phone home with your credentials, and in principle run your own version of it.
What this does not automatically mean is that the company behind it doesn't also run a hosted service that handles your email on their servers. Several products are open source in the client sense while still routing your email through their infrastructure when you use their hosted service. Open source is a statement about the code, not necessarily about the data flow.
For email specifically, the relevant question isn't just "is the client open source" but "does my email content pass through your servers." These are separate questions with separate answers for most products.
Thunderbird is the most established open source email client. Mozilla has maintained it for over twenty years, and it's genuinely capable. It supports an unlimited number of IMAP accounts, handles multiple identities and send-as aliases, and your email stays on your device and your provider's servers. Nothing goes through Mozilla.
The honest drawback is that it shows its age. The interface hasn't changed much in a decade and doesn't look like any other software you use in 2025. For someone managing multiple businesses, the multi-account experience is functional but not designed around that use case. Your accounts live in a sidebar, you switch between them manually, and there's no concept of grouping accounts by business context.
If you need something free, open source, and reliable that runs on your own machine, Thunderbird works. It just won't give you the workflow you'd want.
Mailspring is an open source client that looks significantly more modern than Thunderbird. The core application is MIT licensed and available on GitHub. It supports multiple accounts and has a cleaner interface.
The complication is that some Mailspring features require an account with their service, and historically the business model has been uncertain. The project has had periods of unclear maintenance. If you're evaluating it as a long-term option, it's worth checking recent commit activity and community health before depending on it.
On Android, K-9 Mail was the standard open source email client for years and was recently acquired by Mozilla to become the basis for Thunderbird on Android. It's solid, actively maintained, and handles multiple accounts well. The interface is utilitarian but functional.
This is genuinely a good option if your concern is specifically about a mobile client that doesn't exfiltrate your data. For desktop use, it doesn't apply.
The common thread across all of these is that they were built for someone managing personal email, maybe with the addition of a work account. None of them have a concept of organizing accounts by business context.
In practice, this means that if you have four accounts across three businesses, your sidebar shows four accounts in a list. You switch between them manually. There's no grouped view that shows you everything from your Agency while hiding your SaaS inbox. There's no concept of a spam domain that's visually isolated from your real business inboxes. Compose identity is something you set by selecting an account, not by being inside a business context.
For a founder running one company, this is fine. For someone running two or three, it's a meaningful gap. The separation between businesses that you need for clear thinking and identity discipline requires workflow support that none of the existing open source clients provide.
The mainstream alternatives, Spark, Canary, Superhuman, cover the multi-account case better from a usability standpoint, but they route your email through their servers. When you use their mobile apps or web interfaces, your email passes through their infrastructure.
This is mostly fine for people who've already accepted similar arrangements with Google or Microsoft. But for founders who are deliberately using providers like Fastmail or self-hosted solutions because they want to keep their email away from large platforms, moving to a hosted client defeats the purpose.
The only architecturally clean solution is a client that runs locally or on infrastructure you control, connects directly to your IMAP providers, and doesn't pass email content through any intermediate service.
Cereal was built open source from the beginning specifically because privacy claims without code are meaningless. The full stack, including the IMAP sync engine, the API, and the dashboard, is published under AGPL on GitHub. You can read what it does with your credentials and your email content.
The self-hosted version runs on your own machine. Your IMAP connections go directly from your device to your email provider. Nothing passes through Cereal's infrastructure. If you want to verify this, the code is there.
The hosted Pro version is honest about its tradeoffs: email passes through Cereal's servers in transit so the web and mobile apps can work, but email bodies aren't stored permanently. The metadata necessary for a fast interface is encrypted at rest. The same code that runs the hosted service is the code on GitHub.
Beyond the privacy architecture, Cereal was built around the multi-business use case rather than the single-user case. Each business is a separate context in the dashboard. The spam domain is a first-class feature, not something you approximate with a filtered label. Compose identity is set by which business you're working in, not by a dropdown you have to remember to change.
The short version: Thunderbird is your best current option if open source and local-only are non-negotiable and you can accept an older interface. K-9/Thunderbird for Android is the right choice on mobile in the same situation.
If you're willing to trade some open source purity for better multi-business workflow, and you want to read the code before trusting it, Cereal is worth evaluating. Start with the self-hosted version, which makes no demands on you to trust anyone.
For most founders, the hosted Pro version is the practical choice: you get the workflow, the mobile app, and the multi-business organization without running your own server. The tradeoff is that email passes through Cereal's servers in transit, and whether that's acceptable depends on your threat model and how much you trust the published code.
That's a decision only you can make, which is exactly why the code is public.