The problem I have is if wan 1 drops then all traffic is directed over wan 2 and this is causing email to be dropped due to the dynamic public ip being blacklisted.
You could try this ,
1: configure a loopback interface with some bogus address
2: config a PBR with the src being the mail server or whatever
3: set output device to be the loopback
config system interface
set vdom "root"
set ip 169.254.1.1 255.255.255.255
set type loopback
set snmp-index 999
config router policy
set input-device "port1"
set src "18.104.22.168/32"
set output-device "lo0"
Or that might be to extreme,you could just disallow it via a FWPOLICY ;)
A firewall policy comes next after the route lookup
config firewall policy
set srcintf < interface where mail server set>
set dstintf wan2
set srcaddr < my server1 > < my server2 > < my server3 >
set dstaddr all
set service SMTP
set action deny
set comment " drop mail from wan2 since wan2 is blacklisted due to be on a RBL blah blah blah"
set schedule always