I'm also having a problem with posting by email, only it's waaay more complicated than visiting the -mail.php page:
I set up my post-to email address, with an alias (so I have a DH m###### username and an alias email address like "firstname.lastname@example.org").
I entered mail.dreamhost.com as the server, with port 110 as the port. I originally tried mail.mydomain.com, but that didn't work either.
I sent an email to email@example.com and verified that I could receive it, both through webmail and by creating a POP3 account in my email client and checking the mail. I sent another test message, so I'd be sure there was a new, unread message in the inbox.
I visited wp-mail.php. I get "There doesn’t seem to be any new mail." I get this whether I change the mail server to mail.dreamhost.com or to mail.mydomain.com. I get this whether I change the username in WP to the m##### address or to my alias (incidentally, emailing to the m##### address results in the mail being lost somewhere in the ether).
I am completely befuddled. The email account works just fine. I can get email there. I can send email to there. I'm checking wp-mail.php. But no mail posts.
Most of the troubleshooting for WP post-by-email tells you to check wp-mail.php and to add an exclamation point if you get some weird error message involving "noop," but I don't get that error (unless I add the exclamation point).