I have a few POP email accounts set up on my Desire and have occasionally had problems when sending - failed send etc, often when using home wi-fi but not always. I had set up the accounts using the settings on my email providers support page.
The problem turned out to be the port number in the SMTP (mail out) settings. My email provider gave me 2 options for the port number. Whenever I get a failed send, or if the email sits in the outbox for long periods, changing the SMTP port number from one to the other always solved it.
This solution may not work for you, but the symptoms of the problem seem exactly the same so it's worth checking what your email providers settings should be and see if there are any options that you could try.
Another thing I discovered was regarding manual set up of POP email accounts. By default, you enter the incoming (POP) server settings first. When the screen appears asking for the outgoing (SMTP) server settings, some of the text fields are auto-completed using info you entered for the incoming server. The auto-complete can be very inaccurate, and the errors are easy to miss. make sure you check every text field before proceeding to the next screen.