Thanks for all the advice.

Recent tests have revealed that this situation has been resolved. I
wish I could explain how this has been achieved, but I haven't got the
slightest idea.

I tested jpg, pdf and txt attachments by sending e-mails to myself and
they were all successful.

Just one idea. Is it conceivable that the PDF attachment which sparked
off this issue was corrupt and caused some (temporary) damage?

Just hope it works tomorrow!

Merry Christmas.

TC