Finding trace of deleted message

> Months ago I think I might have accidentally deleted an email that I now 
> need proof that I received. I don't suppose there is any kind of log 
> file that might prove I had received it is there?
> Historical Logging is switched on. :-)

OK, so, assuming the logging keeps data for far enough back, it should at least tell you that you received the message, but it won't hold the message content.

> was the sender

Assuming '' was in the sender's email address somewhere, then:

Go to Settings -> Database -> Query and do
SELECT subject, fromaddr, datetime, size, name FROM logdb.messages INNER JOIN logdb.recipients 
USING (msgid) WHERE fromaddr ilike '' ORDER BY datetime DESC;
That will show a list of all the messages containing '' in the sender's email address.

If you know a part of the subject, then you can add:  AND subject ilike '%<text>%'
If you know, say, the earliest date it could be, then you can add: AND datetime >= '2015-01-01'
(add these just before the 'ORDER BY')

Another thing to try, if you're using a recent version of VPOP3 is Diagnostics -> Message Search. This will find a message which is anywhere in your message store, so you can see if it's in an unexpected folder or something. If you deleted the message then it won't appear there, but it may be that you didn't delete it, just filed it somewhere.

Also, if you deleted it since having VPOP3 6.17 or later, then it could be in the message audit log. This shows actions performed on messages. (The message audit log may not be turned on, check in Settings -> Database -> Message Store for the "Enable message audit logging" option)

in Settings -> Database -> Query, do 
SELECT * FROM msgaudit WHERE sender ilike '' ORDER BY id DESC;