Over the last 2 days I've noticed that I am unable to send mail; my mail gets stuck in the Outbox and the only solution to the problem that I've found so far is to Quit Outlook and restart it-then (with luck) the mail in the Outbox sends. This does not always work though.

My mail provider is BT (in the UK): @btinternet.com

My version of Outlook is in screenshot below. I've also attached the error messages that Outlook provides me in the "log".

The only changes I've made to Outlook recently are the Office updates (2 days ago) and I've changed my settings on a gmail account to have my mail forwarded to my btinternet.com account (but I don't think the latter has had any impact on this "send" problem-it is just a completely unrelated task). 

Can anyone suggest any possible reasons this is happening?

Many thanks!