Thanks to all who replied.
I checked in the group's activity tab and didn't find any record of the messages being received no matter where I looked. I just emailed a test message to the group and that went through okay. I then tried resending the failed message and got the same 500 error.
The message includes two attachments, that may be the problem. The first attachment is a 10-page PDF. The second attachment is hard to open, has no file extension and is not recognized. I finally managed to open it with Firefox and it appears to be a text version of the PDF. But with the text all scrambled together.
I then tried sending a test message with just the PDF attached and that went through routinely. The member in question sends a lot of attachments, Lately he's been sending some that have no file extension. I don't know what they are or how he is creating them. Like many members, he is not very forthcoming or responsive to questions.
tommy meehan
Tommy Meehan