Can Nginx route SMTP based on login credentials?

[spockdude]

I have several users using the same mail host (smtp.example.com) to send
outbound email using authentication.  I would like to split it up into
several outbound servers and assign specific users to specific mail servers
without having to change the mail host information in each user's email
client.  Can Nginx do this or does it always just spread that traffic
between all backend servers?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,272088,272088#msg-272088