I have a curious email problem that is causing much frustration.  I built a DNN site in 5.6.2 to replace a client's old asp site.  Their mail was routed through a hosted Exchange server provided by another company. In the old site the MX table looked after the routing.  I have copied all of the old DNS zone file settings including the MX table, and just updated the IP addresses for the new host.  I then changed the nameserver to move everything over to the new host and the new site. Initially everything seemed to be fine.  However, then the client noticed that some mail was not coming through to them and I cannot find out why. The exchange server hosting company are adamant that it is not their problem and they can't understand what the problem is.  I can't see any reason why the mail isn't working.  

Does anyone have any ideas?  What should the smtp settings look like?  Can this affect mail in this configuration? Does the host email address have any bearing on anything related to this?  Any constructive suggestions would be much appreciated.

There's nothing untoward in the error log.  I currently don't have any information from the Exchange host about their relay logs, but will try and see that.  There is one particular domain from which mail is not being received and I have had them check to make sure it's not somehow become blacklisted.  I do suspect that it is related to firewall, anti spam, or anti virus settings at the Exchange server, but they maintain that nothing has changed.  Do the smtp settings on the site make any difference, do they interfere with DNS settings, or doesn't it really matter what is there?  I remember reading somewhere in this forum about the host email address had to be the same as the domain, but is that true?

Doug