olemus
meiliserver mis saadab edasi sõnumeid,
mille saatja ega saaja pole kohalik kasutaja
=
a mail server that forwards messages whose sender or recipient is not a local user

ülevaateid
https://netcorecloud.com/blog/smtp-relay-service-guide/

https://www.socketlabs.com/blog/smtp-relay/

https://www.duocircle.com/content/mail-relay-smtp/open-mail-relay

standardid
https://tools.ietf.org/html/rfc2645