james-project

Apache James is a mail server you can program. SMTP, IMAP, POP3, and JMAP work out of the box, but the point is the mailet framework: drop your own Java classes into the processing pipeline and route, filter, or transform every message with custom logic. All of it is free under Apache 2.0, from a foundation project that has been at this for two decades.

Running it is a real commitment. The simple setup (one JVM with local storage) is manageable, but the distributed flavor wants Cassandra, OpenSearch, S3-compatible storage, and RabbitMQ before it says hello. Then add the usual self-hosted email tax: reverse DNS, SPF, DKIM, DMARC, and IP reputation are all your problem. This is infrastructure work, not a weekend Docker project.

Use James when email is part of the product: you're building mail handling into a platform, or you need processing rules a normal mail server can't express. Teams that just want mailboxes should pay Google Workspace or Fastmail and move on. For self-hosted sending without the Java, Postal is the better fit.

The catch: James is a toolkit wearing a server costume. The docs assume Java fluency, the community is small next to mainstream mail stacks, and every custom mailet you write is code you maintain forever.