Originally Posted by Travisimo

I've been using K9 and it works great! If you are using IMAP and delete a message, it will be deleted off of the server and any other device that also has that e-mail set up as IMAP.

For example, I have a MobileMe account that I have set up on my iPad and on my Droid X. If I get a new message, it shows up on both. If I pick up the Droid X and delete the message, it will also disappear from my iPad. The same is true if I first read and delete the message on my iPad.

POP3 is a different story. You can set it to delete a message off of the server, but if you have another device that has already checked mail too before you delete it, it will stay until you delete it from there too. And sometimes it doesn't appear to delete off of the server at all, so I've had mixed results with POP3.

IMAP works great though with both Gmail and MobileMe.