This one stumped me for literally months. And I blamed every system other than SmarterMail, which turned out to be a good thing because I learned more. But here's the skinny:
I use SmarterMail 7.x (may apply to other versions) to host my mail as well as many other domains for my businesses and some clients. Anyway, when signing up with Twitter and entering my email address, I would wait in perpituity for the confirmation email - sometimes hitting resend many times. After a while I would give up and simply use another mail account on a different system - I have Exchange hosted elsewhere, a Live account, and a couple gmail accounts. Those always worked. Well, after a final bout with this, I turned on firewall logging to see if the email was even getting to the server. It was!!! The address was email@example.com. I decided to whitelist this address, but that did not work. After searching through the server settings on SmarterMail I stumbled across this:
ARE YOU KIDDING ME!!! Obviously I knew what to do at this point. Once I deleted this entry, my confirmations began to flow through! Sweet! So to summarize, SmarterMail by default (atleast in this version) blocks any email address with 'postmaster' in the address. Definitely a SPAM defense, but in this case it was more of a headache. This is done at the server level. So if you are a customer of a SmarterMail system, and are not getting your Twitter confirmations, you will need to have the owner of the SmarterMail server make this change. Good luck and here's to a sizzling fix!