I suspect it is a setting somewhere in Outlook that is causing the problem but that is just a guess.
You could contact Microsoft to see what they say but no doubt they will blame BT and if you contact BT they will no doubt blame Microsoft or state that they do not support email clients which means they do not offer help if you have a problem with an email client because there are far to many email clients available to have a good knowledge about them all.
Personally I would bin Outlook and stick with Thunderbird which is a perfectly good email client which I have been using for years on multiple devices without any problems.
