I just posted this in response to somebody having a similar problem that I was having, so I thought maybe you guys would like to know what to do in a situation like myself.
I decided to keep a saved copy of mp3s I downloaded off Amazon just in case my phone gets nabbed or damaged. I thought that the easiest and fastest was to do was e-mail the mp3's to myself. I bought a few mp3's here and there and this process worked out well ... until I bought four at once.
I ended up not receiving e-mail for a few days. I don't know about you guys but my e-mail goes in spurts. I'll get like 5 - 20 in one day, and then nothing for three. It didn't occur to me that I wasn't getting anything until about day 5. Since my Droid came into my house, checking my e-mail on my computer is like a once a week thing now ... unless it's an in depth response I don't need to get online anymore... hence the reason I didn't realize I hadn't gotten anything in quite a while.
Here is what I had to do to delete my message(s) and get back to the norm:
Hit the "menu" button on your droid
Go to "labels"
Go to "Outbox"
I ended up having to delete all the things I've tried to send in the last few days ... including the mp3 e-mails. I found that I couldn't delete by long press and deleting from there, I actually had to OPEN the message and delete it inside the message.
After you're done, back out of gmail and go back in.
Hit the "menu" button
Go to accounts.
You should see your e-mail address... Hit the menu button again
Hit the only button you see "account settings"
Under "Manage Accounts" click on your e-mail address.
Make sure all three are checked and hit the menu button one more time
Hit the only button that you see: "Sync now"
Mine took me doing that and repeating the "backing out of gmail and going back in" steps a few times, but eventually it started working again.
The reason mine kept doing it is because the e-mails were too big to handle all at once (I'm assuming) and got stuck in a sense. Obviously a bit of a bug for V or Android to work out. Maybe in 2.1?