It appears to be something to do with the way Avast's "Mail Shield" interacts with Thunderbird.
As a test case, first without the "Mail Shield" installed, I sent a 1 liner plain text "test" message from one account to another, and it went through, no issues.
Then after installing "Mail Shield", I tried the same - this time, the mail message could not be sent, even after having edited the "tcptimeout" setting as described earlier. Again, the same error:
smtp.office365.com timed out.
One thing I noticed, after installing "Mail Shield" my Thunderbird client went into a resync of all mail messages, however even after that had finished, I still could not send the simple plain text mail message (no attachments, simply one 1 word "test").
The problem only appears to be with sending from Microsoft mail accounts (I have several). If I switch the "From" account to a gmail or POP account then I have no issues.
Looking at the Thunderbird Troubleshooting report, the main difference between the Microsoft Accounts and the others, is that the Outgoing server uses a "Connection security" of "STARTTLS" whilst the others use " SSL/TLS". Perhaps Avast's "Mail Shield" doesn't support STARTTLS ? Surely there must be others who have "Mail Shield" installed whilst using Thunderbird ?
