By any chance are all three using the microsoft office365 server ?

Micosoft now state all email accounts are 'alias' and only the one set up as 'primary account' can send via smtp. You can choose what to set up as 'primary' (done in the webmail microsft account) , but then in Thunderbird you can only use that smtp for all accounts to send - so each account SMTP server settings uses that one SMTP primary account address. So if you do not want replies going to wrong address - set up a 'REply-to' email address in Account Settings'.

Thanks, but no, only the Hotmail one uses a MS server.

If you're still having this problem, please post a screenshot of the error messages you're getting.

Read the following article in the meantime to see possible causes of the problem:

https://support.mozilla.org/kb/cannot-send-messages

Thanks David. I've just tried Thunderbird again and for no obvious reason (i.e. I haven't changed anything) it all worked fine. Most peculiar. Thanks for the 'cannot send messages' link though, that could prove useful in the future.