Use smarthost if master MX fails (DNSBL)
 

I have the following scenario:
I have two running mail servers for our domain (sendmail, slackware). MASTER and BACKUP. Previously it was connected to the Internet by ADSL connection. Here we had low connection speed so we changed the ISP for fiber connection with higher speeds. The only problem is that they cannot give IP for us from static space. This causes that our mail users cannot send mails to several servers which uses SORBS as DNSBL. They just list any IP within the dynamic ranges.
I was trying to argue with both the ISP and SORBS, but without any results.

Now I think I have two options:

1. Go back to the first ISP which can give a clean static IP for us. (Not a good option - low speed, already closed contract with the new ISP ...)

2. Use the smarthost of the second ISP. (I don't like it, but probably that will be the only option ...)

I was think I can solve this problem like this:

Because we have contract with the first ISP for longer time I can use the clean Static IP and connect there the backup server.
MASTER will send all the messages which are not stopped, through the FIBER connection and when the sending fails (because of DNSBL) it uses the BACKUP as smarthost for delivering the mail.

Gives this sense???
Are there any other options for me???