Outlook Version 16.114.1 (26102031) not *sending* mail-BT is my mail service provider

Since webmail sends fine, this is almost certainly an SMTP authentication hiccup between Outlook and your provider's servers rather than an actual conflict. First thing I'd do is open the macOS Keychain, delete the saved password for that account, and re-enter it in Outlook's account settings, since a stale cached credential is the classic cause of intermittent send failures like this. While you're in there, confirm the outgoing server is set to require authentication using the same credentials as incoming, on port 587 with STARTTLS or 465 with SSL. If it still misbehaves, remove the account from Outlook entirely and add it again, which forces a clean handshake with the mail servers.