I experienced this with 6.0.0-3375 . There may have been some network connectivity issues that precipitated the event -- the emails were sent basically as soon as I tried to dismiss the alerts. I've disabled calendaring in Thunderbird in the meantime. I think a mitigation may be to, if there are a bunch of notifications visible that shouldn't be there, quit davmail before touching Thunderbird. But, I have not tested this (and am not willing to, since I've already accidentally bothered too many people...