SMTP Forward Nginx Proxy

[chobit]

Almost all of our customers send e-mail through our private SMTP servers,
but we have one customer who chooses to use a third-party SMTP provider. The
third-party SMTP service requires whitelisting of any sending IP addresses
which is normal.  
Unfortunately the components in our infrastructure which send e-mail are
part of an autoscaling group so the IP addresses can vary.  
To solve for this problem I would like to setup an nginx configuration which
accepts SMTP connections to it and then proxies them to another IP address
(the third-party SMTP service) so the requests to the mail server always
appear to the third-party SMTP service as if they came from the same
server.

Is it possible to solve this issue with ngingx smtp proxy?
How should i  forwarded smtp in case with  third-party SMTP service?

Posted at Nginx Forum: https://forum.nginx.org/read.php?2,263084,263084#msg-263084