I have a client that still uses AOL and have a lot of their email forwarded from their domain to their AOL account. After moving to a new server they were having more emails not make it to AOL. I discovered an issue (typo) that my hosting company had in the rDNS for my new IP, the main IP of the server.
So this got me thinking, does the Qmail Sender IP only work when a user is sending an email themselves, manually?
But when an email is forwarded by the system, it still uses the main IPv4 (or IPv6)?
My logic states that a forwarded email should use the main IP address for sending as it is been sent internally and not hitting the smtp as a domain in itself.
I could be wrong so apologies in advance
I find it good practice to only use 1 ip for email (send/receive), with a reserve IP address in case the main one becomes blacklisted.
I ensure both ip addresses have fully rdns setup, fully tested for rdns and if changing servers, ensure all ip addresses (cidr), are listed for FBL with all providers, and check regularly for blacklisting/reputation, to ensure there in good shape. Obviously, reputation is built up over time and how quickly depends upon email volume.
Lastly, it is always a good idea to ensure your server IW nodeworx SSL is fully trusted, with all chains included correctly, so there is no issue on TLS connections, i.e. Not dropped due to no trust
I hope that helps a little