Help!?

I've been using WLM for approx 18months now with no issues. All of a sudden I'm getting error codes, but only with some mails. Equally, some mails are not getting further than my outbox but with no error code. 

I'm receiving emails OK (I think anyway).

Can anyone help.... please??

(ps - I've read elsewhere that my 'server requires authentication' box should be ticked, it already is. No changes have been made to any settings at all lately. It's just stopped working)