I wrote my own smallish SMTP server. My goal was to create a simple program that would accept and store email without making a huge fuss about it.
I call it ghsmtp.
Implementation language is C++.
License is AGPLv3.
Some of my design goals:
Standards supported include…
Simple Mail Transfer Protocol (SMTP)
Internet Message Format
SMTP Service Extensions for…Message Size Declaration (RFC 1870)
Other standards of interest are…
Sender Policy Framework (SPF)
DomainKeys Identified Mail (DKIM)
Domain-based Message Authentication, Reporting, and Conformance (DMARC)
Message Header Field for Indicating Message Authentication Status (Authentication-Results)
Authenticated Received Chain (ARC)