Although I'm a new The Bat! user (in fact, still evaluating it), I learned of this problem when I first read about The Bat!, and it was one of the first things I looked for, because I have a Gmail account, too. It seems that some mass-mailing programs spoof The Bat!'s headers when sending their spam. Not the first time someone uses a stupid filter to try to catch spam...

The solution: in The Bat!, open the menu Options - Preferences. A configuration window will open. Select the "General" category in the left pane and uncheck the option Use X-Mailer header in messages (for administrative purposes). Click on OK and it's done. The Bat! won't place an "X-Mailer" line identifying itself in the headers of your sent messages any more, and Gmail won't block them.