Open relay.. sure it'll be on the black lists so fast that I probably won't receive anything from it.
Backup mx server ? I used to have my ISP providing one of those.. but you know what ? it was more of a back door for spam than a useful item. since I wasn't hosting the backup system it had no checks in place and would accept mail from anyone anywhere to email@example.com
, then forward all that crap to my main mail server when it came back up which had the secondary mx whitelisted..
Spammers love secondary MX's because they KNOW this happens to people, They intentionally will send their spam to the secondary MX instead of the primary. For some reason SBC did this too, we spent some time trying to figure out why all emails from SBC were delayed a couple hours.. turns out it was a misconfiguration on their side to send to secondary MX first.. not too bright imho
I have spoken with several other mail admins that had this same problem. the resolution ? Dump the secondary MX and allow the mail system to handle outages the way it was designed. If you can't reach destination, wait, retry, if delay is too long (typically a couple days) bounce email.
My users recieve considerably LESS spam now that everything that comes in is filtered, blacklisted, rDNS queried, etc.. etc.. etc.. It's nice to be spam free again.
I'd advise against the secondary mx unless you can implement it properly (user lookup and filtering) and the Open relay idea is even worse imho. If you are going to do it, please email me the IP of your servr so I can put it directly into my local blacklist before you start..