Mailcow is designed for multi-tenant hosting. Each domain has independent quotas, DKIM keys, ACLs, and relay rules.
Per-domain settings that matter:
| Setting | What it does |
|---|---|
| Default mailbox quota | Cap for new mailboxes (override per user) |
| Domain quota | Total disk usage across all mailboxes |
| Max number of mailboxes | Hard cap for the domain |
| Relay all recipients | Bridge mode — Mailcow accepts and forwards on |
| Relay non-existing mailboxes | Forward to upstream for unknown locals |
| Backup MX | Spool mail when the primary is offline |
DKIM per domain: Email → Configuration → ARC/DKIM keys → Add. Always generate a 2048-bit RSA key with a selector like mailcow or s2025. Mailcow displays the DNS TXT — paste it into your zone before sending. DKIM signing only happens once the public key is verifiable.
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