As luck would have it, we sent a forum announcement to our membership, and discovered a bug in the process!
You have probably received an email from MTBS letting you know that there is a PM waiting for you in the MTBS forums. This is correct. However, even though there is only one message waiting in your MTBS Inbox, multiple reminder emails went out for the same announcement.
We apologize for the inconvenience, and have put a stop to the problem while we diagnose the issue. It turns out we are not alone in finding this error, and have come across other PHPBB forum software users who have faced the same issue.
Thank you for your understanding in this matter.