Thanks OF - I got a notification of your post.
Think its sorted what the problem was.
- Mostly the spam filters from Dr_Ts email hosts.
Something to do with because something to do with the way I had the server config for mail to go out -
ie the forum is a subdomain of the main domain name and it couldnt authenticate/tie up the 2 names so thought it may have been spam.
Although lots of forums must be set up like that, I made some changes so that mail sent out from the forum for notifications now has its own reserved address that now ties up with the forum subdomain, rather than being part of the main domain.
This was a rare incident, but I wanted to make some changes to make sure it couldnt happen again.
Whilst on the subject, Just a reminder to everyone that the email address used from the forum server isn't monitored and you cant reply to any email sent from the server.