Did this yesterday but the issue came back today I have done it again this morning
I have still not managed to resolve this. It is only on my laptop. I can send emails from my phone and also my ipad without issues also on the web email system from BT. Anyone found a solution as this is a pain
I provided two links to 2 Microsoft community forum with something that provided a fix for a problem that persisted for 2-weeks. But ultimately I think there is a software incompatibility between BT email server and Microsoft Outlook. I have an Ionos email account on same Outlook application and that has no problem. If you search for my name you should find them.
Below a mash up of a work round to intermittent problem sending emails via Outlook 2016 that works for me. First 3 steps from the Microsoft forum.
1) Open Control Panel by clicking the Windows key and typing "Control Panel"
2) In Control Panel, on the upper right hand corner search for "Mail"
3) Once found, double click on it.
4) On Mail Setup dialogue box click on "Email Accounts..."
5) Assuming BT email address set as default and selected, click on "Repair..."
6) "Repair Account" dialogue box opens. Make sure BT email address and password entered (may come up in any case). Click "Next".
7) Two "Configuring" displays come up. You may see "Congratulations" on the second display. Click "Finish" on second display to generate test email.
For me this provided a fix for a period, until I had to re-run the routine.
There is a software issue with BT systems that introduces an incompatibility with Microsoft Outlook.