''<p>wil47 [[#question-1372993|said]]</p>'' <blockquote>Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: Connection refused - OXSUS003_101 - https://postmaster-oxsus.vadesecure.com/error_codes/#_101. </blockquote> That is not a problem you can solve in Thunderbird. Thunderbird is not stopping the email. The message you received offers you a specific link which tells you what caused the problem and how to fix it. The info at link says: ERROR _101 550 5.7.1 Connection refused '' Vade Managed Service platform protects its users. We could not accept your email because your email provider is listed on the Vade Threat Network list. If you think this blocking is not justified you can fill up a '''abuse request''''' ''This list contains IP addresses which have been identified as sending spam. We use this list to protect our subscribers from receiving spam.'' At the link the 'abuse request' text is also a link which takes you to a form to fill out. You need to follow the instructions you have already received.