I just downloaded Thunderbird to replace Outlook Express on my computer. I setup a new list in the Address book. The details are: List name: SuperSpinners1 List Nickname: Spinner1 Description: First Group I'm using this group to BCC: email a list of under 100 people. This list will work for a bit, but then all of the sudden I'll get error when sending an email with the list in the BCC: "Error when sending group email: Message: 5.5.5 RCPT TP syntax error." It appears that Thunderbird taking one of these detail parameters and thinking it's an email address rather than a list. Or rather, it's attempting to send to "First Group" rather than expanding the list to the actual email addresses, or something like that. I have tried adding spaces to the "List Name" and "List Nickname", removing the description field entirely, and this issue always seems to come back. I've dropped and recreated the list multiple times, and it always comes back to this. I'm using the latest version of Thunderbird (38.4) on Windows 10. I'm a 100% confident of the following: 1) All of the emails in my list are well formed, no spaces, etc. 2) My ISP will allow emails to a list of up to 100 people. I purposely have kept my list under 100 people due to this. This is a maddening issue, and I'm about to throw Thunderbird in the wastebasket and move on to something else. Does anyone have any ideas? THanks.