I am hoping somebody can help me out. I am not a DNS expert and would like some input about the following scenario.
We host our own exchange server and I am in the process of testing routing our email through a 3rd party spam filtering company.
To do this I was required to create 4 mx records for my domain directing the mail to the 3rd party servers where it is filtered and then forwarded to my in-house exchange server.
The 4 mx server records all have a different priority.
My dilemma. The 3rd party company requires that all email addresses be listed on their servers. This in itself is not a problem unless for arguments sake I create a new user on our server and forget to duplicate it on the 3rd party server. Obviously, email sent to that address will fail at the 3rd party server.
My question is, If I created a fifth MX record pointing directly to my server and put it last in the priority list, "Will the email get through" Am i right in thinking that once the first 4 have failed (Due to the address not being listed on the 3rd party servers) that the last server, ie -Mine will be tried with success. If so I understand that the emails won't be filtered for spam but at least I won't be losing emails.
Also would this affect the delivery to the 3rd party servers for valid email?