I use Touchdown as my Exchange Client. Download from the Market. You get a 30 day free trial. I bought it and I love it. A very powerful application that makes my Epic my office when I am out of the office.
Very easy set-up. Give it a try.. hopefully it will solve your sending email issue.
I'm having this same issue. OP have you found a solution?
For some reason I am able to receive emails through my exchange account but when I try and send emails they just hang out in my outbox. Someone I work with also has an epic and has no such issues. Gmail works fine and i have a strong connection, any ideas?