Consider querying with stevenage.gov.uk why their DNS record lacks an A record; this is likely causing the failure observed in Thunderbird.
FYI: the reason why the issue does not occur in webmail is likely due to its use of a different SMTP server that does not replicate the same checks as those performed by the server email clients use.
--
I'm a Very Insightful Person, I'm here to share knowledge, I don't work for Virgin Media. Learn more
Have I helped? Click
Mark as Helpful Answer and solved, or use
Kudos to say thanks