I would say hat if you keep rending the same thing over and over and sometimes it works that the problem is not in Thunderbird, but with the server accepting the mail or something sitting in the middle, like an anti virus scanner. This post has good advice on obtaining a log that might shed light on the problem. http://forums.mozillazine.org/viewtopic.php?f=39&t=3073136 My guess however is someone with your provider is setting up a new spam rule, perhaps one that rejects mail from your IP address because you send to much mail, or the address is a domestic one and they have made a poor decision to not accept mail from their customers IP addresses, only their own web servers or something like that. I am seeing more and more poor decisions being made by mail administrators that assume outgoing mail only comes from web mail.