Wow what a fantastic response.
I only used fred to protect the innocent
Or dumb (me)
Perhaps I did not explain very well. fred@fred.co.uk is local, and it is hosted on the SME as the main domain fred.co.uk
there is a Pseudonym fred.smith which goes to user@fred.co.uk
There is a CRM that sends out email directly to the ISP smarthost (Who allows relaying if you are on one of their IP's)
This was setup by the company who supplied and support it.
If I email directly from an external email address, fred or fred.smith works. And so do replies playing email tennis.
However when the CRM sends and email, with the sender as fred.smith@fred.co.uk the email arrives, but when the recipient tries to reply they get bounced?
Did that make any more sense?
Will work my way through what you have suggested and get back to you. Thank you so much for taking the time to give such a through reply.
Best wishes Paul