i got 29 ips im dedicate server
b4 this i try 1 by 1 ip sending email ... dkim pass , spf pass..
then i rotate ip use run this :
# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth0 -m statistic --mode nth --every 5 -j SNAT --to-source 202.XXX.XX.2
# iptables -t nat -I POSTROUTING -m state --state NEW -p tcp --dport 25 -o eth0 -m statistic --mode nth --every 5 -j SNAT --to-source 202.XXX.XX.3
to all my ip.
then rite now spf softfail