go-mail is Email in a Go module.

• SMTP Client and Server {RFC 5321} (with {RFC 8314} and without TLS)
• ESTMP Extensions (8BITMIME, AUTH, STARTTLS, ENHANCEDSTATUSCODE, PIPELINING, SMTPUTF8)
• DKIM {RFC 6376}
• POP3 {RFC 1939} (with {RFC 8314} TLS)
• IMAP4 {RFC 3501}

Easy to integrate, it uses Go closures to handle each step in the email process.

The SMTP server supports DKIM, meaning spam is already accountable to the sender's domain.