That is perhaps when your mail provider, or the complainants changed something. If one person on the list got the mail, it was correctly received by the sending SMTP server from Thunderbird. However lots of providers block BCC in some way, and others block BCC with nothing in the TO: address, yet others just block some mail from some domains without offering a reason. I suggest you look to using the mail merge addon to send customized mail to each individual instead of the current BCC process. I think your results will be much much better. https://addons.thunderbird.net/en-us/thunderbird/addon/mail-merge/ It has a bit of a learning curve, but it is really quite good.