The Send button should become active as soon as you enter an address in To:. Does the same problem occur in safe mode? Hold Shift when you launch TB.

Yes, the same problem happens in Safe Mode. And yes, I know that the Send button should become active as soon as I enter an address in To:. But it does not. That's the problem.

Please try resetting that toolbar.

• Close all current opened 'Write' windows.
• Open a new 'Write' window and right click in the Composition toobar and select 'Customise'.
• Click on 'Restore default set' button and then click on 'Done'.

Toad-Haal, thanks for your suggestion, but unfortunately that has not solved the problem - Send button still remains greyed out!

Is the from showing as a fully qualified address?

Yes, the from showing as a fully qualified address. In fact everything else is showing as normal. It's just the Send button that isn't, it remains greyed out under all circumstances.

As Safe Mode had no effect, this rules out addons etc. Resetting the toolbar also rules that functionality out.

So I'm now wondering if you have something wrong in the Account Settings for that account email address.

• Right click on mail account in Folder Pane and select 'Settings'.
• Carefully check everything for the account as I think something is wrong. Don't forget to check the selected outgoing server (SMTP) for that account email address as well.

Well, problem solved, but I don't know how! Went through account settings as suggested, couldn't find anything wrong. Then tried Write new email, started to enter an address in To:, and lo and behold, the Send button was active.