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Thank you for your interest Albags, we seem to have different information. Here is a copied and pasted extract from OE Help, listed under 'size limit for messages':-
"Many e-mail and news servers limit the size of the messages you can receive and send. Usually this limit is one megabyte (1 MB) per message, including all attached files."
Yes, OE was receiving/sending before the incident occurred. The problem is only with receiving, not with sending. I checked Security as you suggest but 'Do not allow attachments' was not ticked.