I'm on the Ohio server which was upgraded and restored from backups about 24 hours ago.
Some time after the upgrade, I found that I was no longer able to send mail. I got some kind of SMTP error (the mail client, Thunderbird, didn't give me an error number). Strangely, I could still receive mail, and I could even send mail from webmail.
Anyway, after fiddling around for a while I did a couple of things, one of which seemed to fix the problem.
1. Deleted a mail.mydomain.com subdomain which I had set up to redirect browsing to the horde webmail login
2. Retyped my mail password into the POP account settings in Cpanel (not a new password, I just retyped the old one).
I guess one of these fixed it, or it just happened to start working again at that time.
Don't forget that in your mail client, your user name should be the full email address, like
yourname@youraccount.com, and the SMTP server will work if simply set to youraccount.com (no mail. or smtp. subdomain necessary). In my mail client, you don't need to specify port 25, either.