As we discussed earlier, Gmail can hold years of "read" mail. Especially if the account is being used for archiving reasons, like with your's truly. In my case, Gmail's "recent" command comes in very handy. Just adding "recent:" to the username (as in "recent:[email protected]" ) will show the latest mails only.
BUT: MWM will not allow any editing of the username that's been generated, based on the Gmail address.
This renders manually adding a Gmail account useless.. 
Workaround: make the necessary alterations in MWP and then hit the SYNC button. That'll work, but IMHO MWM should rather use "the Royal way".... 
..
Have to wonder too why the server address appears to be treated the same way.