The first question has to be blacklist by whom? Thunderbird does not do any form of blacklisting, so I would assume you are getting some form of message on sending the email or some form of reply to mail you send. I have seen a number of posts here from various US based ISP'srecently that blacklist any internet address except those they issue from connecting to the internet using their service from accessing their mail servers, others get sensitive about their sending servers. In those cases using a hotspot, any hotspot, be it wifi, bluetooth or a VPN will see you blacklisted until you connect using one of the providers IP addresses. So without any specific information I have ventured a guess. If you want to try and get something specific, you might want to share the exact error message you receive and how you get it. I know about you and your problem exactly what your tell me, and so far I have iut you are getting blacklisted. So what is the IP address that is generating the error (it is usually in the error message) and who is the mail provider (the company whose names comes after the @ in your email address usually.