Thanks for that Charlie.
Charlie Brady wrote:
"Do you have have a "sort header" defined in your mail retreival configuration? If not, then adding one may help with correct delivery, if a suitable header has been added by your ISP."
I realise that multidrop is unreliable. The boss-man wants it that way.

In the past, using SME 5.0, ordinary email messages have gone through to their correct recipients. Mailing list messages have gone through to the administrator who then passed them on to their intended recipients. The default sorting method has been used. I have suggested using the "Standard" method of email retrieval, but the boss-man doesn't want to fork out the money for a permanent connection with a fixed IP address.
My problem is not that emails are going through to the incorrect recipients, it is that mail is not getting picked up at all from the Optusnet mailbox.
The server is being run as a "Private Server and Gateway". It has always run this way. Do you think that it would help to run it in a different mode?