Sean Wallbridge
One thing I’ve done to our own mail to help curb some of the crap spam we are getting, is that I’ve updated our MX record. I’ve done this because we use DYNDNS for a ‘secondary/store and forward MX’ delivery agent. I.e. if our mail server was unavailable (Internet Down, Exchange down, etc.) – it ‘holds’ our mail and delivers it to us when we come back online.
The problem with these types of services is they don’t generally have very good (or any) anti-spam measures. Many spammers have figured this out of course, and as one of their techniques, if they find a mail domain with more than one MX record, they make the assumption that the 2nd domain might be one of these store and forwarder services, and force their mail to go through it – which it then in turn, delivers email to your mail server, thus increasing the amount of spam.
So, the solution is to add another MX record, that duplicates your primary MX record (the lowest ‘cost’ is the primary mail delivery host), with the ‘highest cost’. That way, when they try this technique, they will use our mail server as the first MX and the last, both of which have the appropriate protection.
As per the example above, the host ‘mail.itgroove.net’ is the primary mail server (Exchange) with a cost of 10, our Store and Forward hosting provider is the cost of 20 and the new added mail host is ‘mail.itgroove.net’ again, with a cost of 80 (highest cost).
Note, if a client isn’t using a store and forward service, this doesn’t apply.