Well, we learned how to configure Sendmail to send to a specific IP address on a per domain basis. Google and it’s business service e-mail is now by default publishing IPv6 addresses for MX records, it’s almost impossible to do a per domain setup. We do not want to disable IPv6 entirely on the server, but Sendmail keeps sending out via IPv6. How the hell do you make it stop! It’s quite simple but just a refresher since this is a configuration that is out of sight and mind. Remember that Sendmail is really two things. A Daemon (the part that listens for incoming mail) and a Client (the part that sends/relays e-mail). Naturally you have the DAEMON settings by default, but not the CLIENT settings. So here we go.
I have only found one forum post regarding the proper solution to this problem. It appears not to be properly documented and possibly this is changed behavior in a recent update. However, it does make sense. Essentially you need to tell the IPv6 stack to use your IPv4 address.
Edit /etc/mail/sendmail.mc
Look for
DAEMON_OPTIONS(`Port=smtp,Addr=xxx.xxx.xxx.xxx, Name=MTA')dnl
Below the above line add this;
CLIENT_OPTIONS(`Family=inet6,Addr=::ffff:xxx.xxx.xxx.xxx')dnl
Save the file, make the db files and restart Sendmail.
Now, Sendmail will use IPv4 for it’s CLIENT operations.