Thanks for your reply but I still get the same error message.
Likewise, if I change the delivery address from
info@mydomain.co.uk to my gmail address, it all works fine.
It's almost as if it's a DNS issue, or a problem with mail aliases within sendmail / postfix / whatever the MTA is.
If it's any clue I don't use webmail - I have one MX record that points to my local server here.