[Zope] kicked off list

Every so often, my other email address gets kicked off the zope list for some reason. One day I'm receiving messages and the next day I'm not. After this happened 3 times, I joined with a second address (this one) so I could be sure it was a phenomenon with a single email address. Can anyone give me a clue why this might be happening? I only occasionally send email to the group, and I never cross-post. Thanks!