Generally I simply delete posts with this much misinformation, but in this case, since it's been a few days and some folk may have read it and rely on it, I thought it better to point out why it's incorrect.
For one you shouldnt use 0 as a mx number. Use like 5 and 10.
This is simply incorrect. You can use integer equal to or greater than zero (I don't know if there's an upper limit). These are what we call
cost values. The email server that costs the least will be tried first. We often use the 0 (zero).
Also there is no reason it would bounce if it is setup correctly. Did you add all the domains.
I'm guessing (I hate to do that) that you mean setting up all the domains in DirectAdmin. This is never a good idea; you'd have to check both systems every time you check mail.
The correct way to do it would be to simply create a new flat file to identify the domains for which you're doing this, and a delivery method in the exim.conf file.
Not that I recommend it, but I'll discuss that in my reply to the original poster.
Also I dont know why you are doing this because once your main server is back up its not like its gonna forward the mail back to the master server.
It absolutely is if done correctly.
So your users will have to login to both boxes to get their emails.
See above, and my next post, to the original poster.
Jeff