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There is no verified, comparable three-way production test showing that mailcow, Stalwart, or Mailu is the proven choice for 10,000 mailboxes. The clearest scale-specific account reviewed is an operator’s self-reported Stalwart deployment at a university. The author says they did not run mailcow or Mailu at that scale, so the comparison is a useful case study—not a benchmark or proof that one product handles a particular load better.
For an organization choosing a platform, the practical answer is to compare how each system fits its operations, then pilot it against a representative workload. Mailbox count alone cannot establish capacity or resilience: concurrency, message volume, mailbox size and retention, search, antivirus, groupware, storage, backup, and recovery all matter.
What the 10,000-mailbox evidence does—and does not—show
The operator comparison published by Fanpino on September 23, 2026, reports that its authors run Stalwart for a university with 10,000 mailboxes. That is a first-person deployment claim, not an independently verified result. The article says the authors did not operate mailcow or Mailu at that scale and describes those platforms from their documentation. It therefore cannot establish a head-to-head winner, comparative capacity, service-level performance, or what happens when a node fails.
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1Scan for outdated or missing drivers - takes under a minute2Repair Windows errors before they cause bigger problems3Fix the driver behind crashes, sound loss and screen glitchesThe reviewed material does not provide a common workload definition or comparable performance results for all three products. A count of provisioned mailboxes is not a load test: 10,000 mostly dormant accounts pose a different workload from 10,000 people checking large mailboxes concurrently, running full-text searches, or using groupware features.
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How the platforms differ operationally
| Platform | Documented operational shape | What the available evidence does not establish |
|---|---|---|
| mailcow | A Docker-based suite of separate services, including Postfix, Dovecot, MariaDB, Redis, Rspamd, SOGo, and Nginx, with ClamAV optional. Its web UI covers domain and mailbox administration, DKIM/ARC, TLS controls, quarantine, antivirus scanning, and basic monitoring. | Its small-deployment examples are not capacity guidance for 10,000 mailboxes, and the reviewed material contains no comparable 10,000-mailbox performance or failover result. |
| Mailu | Its setup documentation describes Compose and Kubernetes setup flavors and gives mail-flow and DNS-authentication checks for operators before go-live. | The documentation’s maturity caution is not a comparative production test. No common 10,000-mailbox workload or performance result is provided. |
| Stalwart | Its migration guide describes staged migration from Dovecot/Postfix systems, including mailcow-like installations, using a new deployment, a proxy at cutover, and Vandelay for account data. | The operator’s university deployment is self-reported and asymmetric; the reviewed sources do not provide comparable results for mailcow and Mailu or measured node-failure behavior. |
Mailcow: treat the published minimum as a prerequisite, not a 10,000-mailbox design
Mailcow’s system-prerequisites documentation lists a minimum of 6 GiB RAM plus 1 GiB swap and 20 GiB of disk space excluding email. It also gives examples of 8 GiB for about 5–10 users and 16 GiB for a company with 15 phones and roughly 50 concurrent IMAP connections. These are small-installation examples, not figures to extrapolate into a 10,000-mailbox deployment. The documentation warns that ClamAV and Flatcurve full-text search can be RAM-intensive.
Mailcow’s data layout makes backup and recovery design especially important. The project says email and user data live in named Docker volumes; email is compressed and encrypted, and the encryption key pair is in the crypt-vol-1 volume. Its overview documentation says to back up that volume as well as the other data volumes. A backup plan that saves mail data but cannot recover the required keys is not a complete restore plan.
Mailcow’s prerequisites documentation puts the infrastructure dependency plainly: “A correct DNS setup is crucial to every good mailserver setup, so please make sure you got at least the basics covered before you begin!”
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Mailu’s setup guide says the project has powered “hundreds of e-mail accounts since around January 2016” and “delivered over a million emails.” Those are project statements, not independently audited deployment statistics; the page does not give a separate date for the email total. The same guide cautions that Mailu is “still not massively tested” and says operators should not run a critical mail server before properly testing every feature.
The guide identifies 2024.06 as its most recent stable version and recommends it for new setups, while saying that latest points to the master development branch and should not be used in production. Because this is a version-sensitive recommendation and the documentation labels a specific stable release, check Mailu’s current release information before selecting an image or planning an upgrade.
For a planned go-live, Mailu’s documented checks include confirming inbound and outbound delivery, SPF and DKIM, logs, relay behavior, and DMARC reports. Those checks are useful starting points, not a substitute for testing the full feature set and workload the organization intends to use.
Stalwart: plan migration as a staged change, not a one-step swap
Stalwart’s migration guide includes Dovecot/Postfix systems such as mailcow among its migration sources. It describes installing the new deployment alongside the existing system, preparing a proxy to front public ports at cutover, and transferring account data with Vandelay. Vandelay stores each account in a local archive, so the migration host needs enough free space for the largest mailbox being migrated at one time.
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The guide, last updated August 27, 2026, says Stalwart’s configuration model, administration interface, and storage layout changed substantially after version 0.15. It recommends becoming familiar with the latest release before relying on it in production. Migration procedures and older third-party comparisons should therefore be checked against the release you intend to run.
Best Value
What “holds up” should mean in your pilot
Before comparing products, write down a workload that resembles real use. Include active users and peak concurrent IMAP or JMAP sessions; inbound and outbound message rates; mailbox-size distribution and retention; search and indexing demand; and whether antivirus or groupware features will be enabled. A platform that passes only a mailbox-count check has not demonstrated that it meets those requirements.
Then test the operational risks that a deployment claim alone cannot answer:
- Mail flow and authentication: Verify SMTP delivery in both directions, SPF, DKIM, DMARC reporting, relay restrictions, spam handling, and logs. Include the DNS and relay paths used in production.
- Concurrency and storage growth: Exercise representative clients and accounts at expected peak concurrency. Measure behavior with large mailboxes, the intended retention policy, and search or indexing enabled where required.
- Backup and restore: Restore representative accounts and configuration from backup, including encryption keys and other dependencies. Record the measured recovery time and confirm that restored data is usable.
- Failure and recovery: Test the failures relevant to the proposed architecture—application node, storage, database, or network path—and observe queue durability, service recovery, and safeguards against conflicting state. The reviewed sources do not report comparable results for these scenarios.
- Operations and upgrades: Rehearse account provisioning, monitoring and alert response, patching, security response, upgrades, and rollback with the team that will be on call.
- Migration and rollback: Inventory protocols and groupware dependencies, pilot representative accounts, verify mail flow after cutover, and prove that the existing service can be restored if the migration fails.
Choose by demonstrated fit, not mailbox-count claims
For mailcow, determine how its multi-container service layout and volume-based backup requirements fit your operational and recovery design; its published minimum and small-installation examples cannot size a 10,000-mailbox system. For Mailu, validate each required feature and confirm the stable release you intend to deploy rather than treating the setup guide’s release label as evergreen. For Stalwart, evaluate the current release and rehearse migration if you are coming from a Dovecot/Postfix service.
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One free scan finds every outdated or missing driver and matches the right update for your exact hardware.Free scan · exact hardware matchWhere an organization does not want to operate every component itself, mailcow’s documentation identifies Servercow as a source of professional or prioritized support subscriptions and managed mailcow hosting. Confirm current service terms directly; the documentation does not establish terms beyond identifying those options.
None of the reviewed evidence supports a hardware recommendation for this scale. Host sizing needs to follow a workload-specific design and pilot, not mailcow’s minimum specification or its small-installation RAM examples.
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