Package courier implements various couriers for delivering messages.
Courier delivers mail to a single recipient. It is implemented by different couriers, for both local and remote recipients.
Procmail delivers local mail by executing a local binary, like procmail or maildrop. It is named after procmail just for reference, it works with any binary that:
- Receives the email to deliver via stdin. - Exits with code EX_TEMPFAIL (75) for transient issues.
SMTP delivers remote mail via outgoing SMTP.