Email isn't sending

Hello,

I'm having an issue with Mail. Outgoing emails are piling up in my outbox without being sent. If I quit Mail and then re-start it, the email sends.

I think I've narrowed the problem down somewhat. I have a Roadrunner.com account that suddenly stopped being able to make an SMTP connection, despite my having set all the parameters correctly. It appears that if I try sending an email to this account, once it gets in my outbox, outgoing mail jams.

Does anybody have any suggestions about what is going on, and more importantly, any possible fixes?

Thanks,

Carlisle