I have now got to the bottom of this.  You confirming that it was not a problem at my end has forced me to look further.

In fact the messages I thought were all the same were in fact sent at slightly different times.  They were generated in a frenzy last night at three of my clients as error messages when certain machines failed to connect to my FTP server.  They were sent via Demon who was the ISP for three of my clients.

For some reason, these messages were not delivered immediately but rather, very slowly throught the day.  A few straglers are still coming in now.

I have now set up the client machines to post direct to my server and not via their ISP.

Thanks again.

This started yeaterday.  From early morning till mid-afternoon I kept receiving the same email again and again.

From the headers it was clear that the same email was being forwarded by Gradwell again and again as if the connection was being lost or the delete command was not being issued.  

I logged the issue with them and a while later it all stopped - though they do not claim to have done anything.

Today it has started again with 3 emails, this time being forwarded from Demon.  So the sourse of the problem seems like at my end.

I have turned on session logging for MercuryS and I can see the full conversation for each session.  I expected to see a DELE command at the end of messages received but there is none.

However, even the messages that are not repeated do not show the issuing of the DELE command - for some reason it is not being logged.  All the conversations end as below.

 Where do I go from here?  Are there any other logs I could look.

I need to do something quickly.

13:50:33.174: >> .<cr><lf>
13:50:33.184: << 250 Data received OK.<cr><lf>
13:50:33.234: >> QUIT<cr><lf>
13:50:33.234: << 221 Dessergroup Service closing channel.<cr><lf>
13:50:33.244: --- Connection closed normally at Thu Dec 18 13:50:33 2008. ---
13:50:33.504: 

If the messages are delivered by SMTP there should be no DELE command, you are probably thinking of POP3 which is quite a different matter.

The log excerpt seems to show a session that completes normally. If the message is being sent again it will have to be checked at the sending end, there is nothing you can do about it in your server.

/Rolf 

You are off course right - my brain must have been out of gear.  I'm used to using POP3 and have only switched to having my own server about two months ago.

I was expecting to see some explicit comand that confirmed to the sender that the message was received.

It still seems a bit of a concidence that yesterday it was an email being forwarded by Gradwell giving me this trouble and today it is Demon.  That to me implied that the fault is at my end somehow not properly confirming receipt.