Currently have my mikrotik configured with one wan interface called "wan", I've been assigned a public IP address/subnet which is a 30 subnet...so just the one usable IP address. My router is set to NAT/Masquerade out interface "wan".

My ISP will now issue me a few more IP's, so from a 30 subnet I will go to a 29 subnet. I will be using one of these new IP's for an email server, I want to dedicate this IP for email use only, no internet traffic should go via this IP.

My questions are, how do I assign two IP's to the same interface, and how do I NAT from both but only allow smtp from one...and the rest of the data from the other IP....the NAT is done on the interface name so not too sure how I tell it to differentiate.

Any help much appreciated!

if you have ip addresses from same subnet - then there will be issues - just assign address to the same interface.

for SRC nat you have two options - masquerade where source address is chosen automatically, ir you can use action=srcnat and set to what address packets passing should be source-natted.

Thank you Janisk, I see what you mean about the "action=src-nat".

But how do i seperate the traffic so that internet traffic only goes down IP1 and SMTP only goes down IP2?

i currently have PCC load balancing across 4 wan links, I dont want to create a mangle rule that disrupts this. So the end result will be to have 1 of the 4 wan links configured with a 29 subnet, I will then continue splitting internet traffic across all 4 wan links using PCC, but I will have and extra IP assigned to one of the wan links and this will be dedicated to smtp only.

Thanks so far for your help!