WordPress – Slow mail send using WordPress and Sendmail

There are several sendmail tuning parameters that can affect the initial delivery attempt. Have a look at Sendmail Tuning.

The per-connection delay is caused by sendmail wanting to look up its own fully-qualified domain name (FQDN), which failing all else it does by trying to reverse-resolve its external IP address. You haven't given it an FQDN, so it's trying to get by with localhost.localdomain, which it gleaned from the localhost entry in the hosts file (you can see the evidence in some of your Received: from lines). If you give sendmail an FQDN, by putting an FQDN as the first entry on the external-ip-address line of your hosts file, it should speed up all operations.

As for removing some of those Received: from lines, they are added as an RFC requirement each time the message passes from one sphere of control to another. Do you have a very good business reason for wanting to get rid of them? Because if you don't - and usually, even if you do - it's a really bad idea to mess with them.