Open-source alternatives to Google Workspace and Microsoft 365 email

At a glance

Open-source alternatives to Google Workspace and Microsoft 365 email compared
Tool	Licence	Self-hosting difficulty	Who it suits	Strengths
mailcow	GPL-3.0	Hard: Docker is the easy part. You also need 6 GiB of memory or more, DNS records, and an IP address other mail servers will trust	Teams with someone technical who want a complete mail server with a webmail screen	Webmail, calendars and contacts through SOGo, Rspamd spam filtering, optional ClamAV antivirus, a web admin screen, unlimited mailboxes
Stalwart	AGPL-3.0-only	Hard: one program with a setup wizard, but the same DNS and reputation work, and you need to add your own webmail	Technical teams who want a modern all-in-one server and already like desktop and phone mail apps	One program for mail, calendars, contacts and files, JMAP as well as IMAP and SMTP, built-in spam filter, written in Rust, very active project

mailcow: GPL-3.0. Licence checked 31 October 2026.
Stalwart: AGPL-3.0-only. Dual licensed: the Enterprise edition adds features such as multi-tenancy under a proprietary licence. Licence checked 31 October 2026.

I'll start with something odd for a site like this: most small businesses should not self-host their email.

Google Workspace Business Starter is £5.90 per user a month on an annual plan, so a team of ten pays £708 a year. Microsoft's Exchange Online Plan 1 is £3.10, which is £372 a year for the same ten people. That is not much money for something that has to work every single day.

People still ask about this, usually because the per-mailbox bill keeps growing, or because they have dozens of shared and role-based addresses and resent paying for each. So here are the two open-source mail servers I'd look at, and an honest view of when they make sense.

mailcow

mailcow is the "complete kit" option. It bundles a set of well-known open-source mail programs, each in its own Docker container, and puts one web admin screen over the top.

Webmail, calendars and contacts through SOGo, so people have something to log in to from a browser.
Rspamd for spam filtering, which learns as it goes.
ClamAV for antivirus, which you can turn off to save memory.
Postfix and Dovecot doing the actual sending and storing. Both are long-established.
A web admin screen for adding domains and mailboxes, so you aren't editing config files for everyday jobs.

It is not light. The documented minimum is 6 GiB of memory plus 1 GiB of swap, with 8 GiB recommended for about 5 to 10 users, and 20 GiB of disk before you store a single email. It can't run on OpenVZ, Virtuozzo or LXC, and the docs specifically warn against a Synology, QNAP or any other NAS.

Add the port list (25, 465, 587, 993 and more) and you can see why a proper virtual server is the realistic home for it.

Stalwart

Stalwart takes the opposite approach. Rather than a bundle of separate programs, it is a single program written in Rust that handles the lot: SMTP, IMAP, POP3, JMAP, calendars through CalDAV, contacts through CardDAV and file storage through WebDAV.

One program to run, on Linux, macOS, FreeBSD, Windows or Docker.
A setup wizard on first start, which asks for your hostname and first domain, then gets a Let's Encrypt certificate.
The modern email security standards, including DKIM, SPF, DMARC, DANE and MTA-STS.
A built-in spam filter, so there's nothing extra to bolt on.
A very active project, which says it ships a release every week and has had a security audit.

The big catch for a small business is webmail. Stalwart's own material describes a web admin screen and a self-service portal, not a mail client for your staff. That's fine if everyone uses Outlook, Apple Mail or Thunderbird on IMAP. If they expect a browser inbox, you need to add one, such as the third-party Bulwark project.

About 85% of the features are in the free Community edition. The Enterprise edition covers things like multi-tenancy, SCIM, live telemetry, AI spam classification and account archiving, most of which a small firm can live without. Archiving is the one to think about if you have record-keeping duties.

What it costs

Google Workspace and Microsoft 365 email

Google Workspace Business Starter: £5.90 per user a month, billed annually. Checked 31 October 2026 (source).
Microsoft Exchange Online Plan 1: £3.10 per user a month, billed annually. Checked 31 October 2026 (source).
Microsoft 365 Business Basic: £5.40 per user a month, billed annually. Checked 31 October 2026 (source).

Self-hosted, run for you

No per-user fees. Adding people doesn't add licence costs.
Setup: a one-off fee to set it up and move your data across.
Hosting and care: a fixed monthly fee covering every app on your server.
I'll quote once I know what you need.

Paid tool prices exclude VAT unless stated.

The UK angle

Be practical here. Google and Microsoft are fine for most small firms, and I wouldn't move your email just to get it off a US company's platform. UK GDPR doesn't ban that. What it asks is that you know where the data goes and have proper terms in place, and transfers to US firms taking part in the UK Extension to the Data Privacy Framework are covered by the UK-US data bridge.

Self-hosting does give you a neat answer on where your mail lives, because it's on a server you chose, in the UK if you like. It also moves the work to you. If the server is breached, that's your problem to find, fix and deal with, not Google's.

Where it might matter is if your customers contractually insist that data stays in the UK, or if you handle especially sensitive information. Even then, check whether your existing provider can meet that before you build anything.

Honest drawbacks

Getting mail delivered is the real job. Installing the server is the easy bit. Keeping your messages out of other people's spam folders depends on DNS records, reverse DNS and your server's reputation. Stalwart's docs warn that a blocked port 25 or missing reverse DNS can stop delivery or get mail rejected.
Some hosting providers restrict mail. Check that yours allows port 25 before you pay for anything.
Downtime is your downtime. When the server falls over on a Saturday, nobody is coming to fix it unless you've arranged that.
Backups are your job too. Email is usually the only copy of a lot of important records.
Spam filtering takes care. You'll be tuning what Google and Microsoft do for you at huge scale.
You lose the suite. Paid plans include other things: Business Starter comes with 30 GB per user of pooled storage, and Business Basic bundles other Microsoft services. Replacing those is a separate job.

Which one?

My pick is mailcow, for anyone determined to do this. You get webmail, calendars and spam filtering in one install, and the people using it have a screen to log in to. The main drawback is the one that applies to any self-hosted mail: getting your messages reliably delivered is hard work, and it never completely stops.

Want the complete kit with webmail: mailcow.
Technical team, happy with desktop and phone mail apps, like the idea of one program: Stalwart.
Everyone else: keep Google or Microsoft. Exchange Online Plan 1 at £3.10 a user is very hard to beat once you count your own time.

If you need email tied into your own systems, such as customer messages logged against the right record, the team at Dev Partners can build that kind of custom software, and it works just as well with hosted email.