I can't begin to tell you how frustrating this one was.
So, local WAMP install on Vista 32bit. Using XpertMailer plugin to use Gmail as SMTP host for outgoing WordPress emails. Something I've done with no problem on my VPS.
On my local machine, I just couldn't get the XpertMailer plugin past the submission 'Loading...' dialogue.
I made sure I could telnet to smtp.gmail.com on the right ports (465/587). I tried everything. Everything! Except making sure that the Open_SSL PHP extension was enabled in WAMP. This post made me think to check: http://www.vishalkumar.in/2009/06/php-mail-using-gmail-smtp-tutorial/
As soon as I enabled it, everything worked perfectly
Posting this because I didn't find the answer using any of my initial search terms. Hope it helps someone.