If you find yourself in a pinch and you need a temporary fix when your primary IP is blacklisted, use the following iptables rule:
/sbin/iptables -t nat -A POSTROUTING -p tcp --dport 25 -j SNAT --to-source [desired outgoing ip]
Keep in mind, however, that you will need to adjust any applicable SPF records for your domains since your e-mail will appear to be leaving via one of the secondary IP’s on your server. Also, remember that this is only a temporary fix - you should find out why you were blacklisted and eliminate that problem as soon as possible. 
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