Consider a situation where the email server ip gets blacklisted in a server with any email server like exim or qmail, we can reroute the smtp via another IP using IP Tables.

For routing the SMTP via xxx.xxx.xx.xx, issue the following command:

iptables -t nat -A POSTROUTING -o eth0 -p tcp dport 25 -j SNAT to xxx.xxx.xx.xx

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