I had an almost exact problem with setting up my Mindspring (Earthlink) email.  I could receive mail on my Droid X without any problems, but all messages I attempted to send just sat in my outbox and the phone would periodically give a notice that it was unable to send the message (Invalid Server Certificate).  After posting this info here, a rep from VZW suggested that I delete the account and start over.  After I did this, it worked without any further problems.  When (or if) you decide to try this, I would suggest that you DO NOT use the "automatically configure account" (or whatever it says).  I think there is where the problem is.  If you manually enter all of the settings, it may resolve your problem.  I used port 25 for my outgoing server and did not select "use secure certificate" or "verify certificate".  

Removing this account will remove any mail that was downloaded to the phone for this account only, but once you re-configure it, you should retrieve them back on your phone.  At least it did for me.

I hope this helps.