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

BT webmail working shows that sending is possible through BT’s website, but it does not prove Outlook’s outgoing-mail connection is healthy. Because both Legacy and New Outlook can fail, there is not enough evidence to call this a confirmed Outlook–BT conflict. During the next failure, capture the exact send error and time, and test a short message without attachments. Confirm Outlook is not working offline. In the account settings, compare the outgoing server, authentication, port, and encryption with BT’s current requirements; Microsoft specifically identifies SMTP authentication and incorrect settings as possible causes. Test the same account on another trusted network to help distinguish a network-specific restriction. Keep webmail as a temporary sending option. Give Microsoft support the build, error, timestamp, and cross-client results, and ask BT to check the corresponding SMTP connection. Avoid deleting the account before protecting any locally stored mail.

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.