Thanks for your help. I thought if the problem is antivrus checking or another security issue then I reckoned McAfee, which I use, should be asked. So I mailed McAfee's customer support and they quickly sent me a link to the instructions as to how to set up email filtering (which I was previously using without problem, but must have needed changing) and so, I did what they said and all is well. The instructions did not match exactly what I had to do - the McAfee screens did not appear in the order they said, but they were easy to follow and get right. The link is:
I hope this helps someone else.
And thanks again.