the documentation has some notes on port forwarding for SMTP. But they
are prefixed with a warning that they are not working currently.

For me this works:
- run Validator's on the IDV host and start its SMTP server on port 1025
- add a redirect rule (IPv4)
iptables -t nat -I OUTPUT -p tcp -o lo --dport 25 -j REDIRECT --to-ports
- point the IDM engine at it (use IP to force IPv4)
dn: cn=Default Notification Collection,cn=Security
changetype: modify
replace: notfSMTPEmailHost

BTW: It seems the IDM engine doesn't like a port with the
notfSMTPEmailHost. So localhost:1025 caused an error.