If you open the Calendar tab and right-click a calendar in the left pane, Properties, is the correct email account associated with the calendar?

Thanks for that idea - sadly it has not helped.

• Calendar tab
• Right-click the only calendar
• Properties
• The email is OK (and there are no other emails listed in the drop-down)

Somewhere, there is a (hidden?) rogue email address...but a different one for laptop and desktop.

Is this a network calendar set up with an add-on, a local calendar, or a network calendar added with the built-in CardDAV option?

It’s CalDAV to Fastmail. My desktop is linked, as is my laptop - but they both are issuing different (non-operational) event non-operational emails….that is why I am sure that the issue is with Thunderbird, not Fastmail.

In the Invite Attendees window, the inviter's (your) address appears at the top of the list. If it's incorrect, select it and start typing the correct address, and autocomplete should display all other mail accounts currently set up in TB. This doesn't explain why the address in Properties isn't entered automatically, but it at least offers a workaround.

Thanks folks. After struggling for at least a week, trying all sorts of fixes on both my laptop and desktop.... a miracle has occurred and all is OK. I'm not sure if Fastmail has some sort of update, or Thunderbird sorted itself (both machines are on 115.3.3 - updated 19Oct23). I really appreciate your help and I am back in love with Thunderbird again. Time for a donation!