SMTP Proxy listen on sub interface only?

Question

We're looking at limiting the smtp proxy listening on all interfaces. Now I can see that it can be limited to x interface/s but can it be limited to a sub interface ie an additonal IP 
 eg. eth0 = WAN 2.2.2.2/27 with addtional IP's of 2.2.2.3, 2.2.2.4 etc 
 Can it be limited to above example 2.2.2.4 or will it just simply listen on all IP's on that /27 subnet?

BAlfson · Answer

Hi Louis, 
 Two NAT rules, in order: 
 NONAT : Internet -> {25, 465, 587} -> {2.2.2.4} DNAT : Internet -> {25, 465, 587} -> External (Network) : to {240.0.0.1} 
 That blackholes all SMTP traffic to the other IPs. Is that what you were looking for? 
 Cheers - Bob

LuCar Toni · Answer

Online Help: 
 Listen Interfaces 
 By default, the SMTP proxy listens on all interfaces on ports 25, 465, and 587 for incoming email traffic. 
 To listen only on particular interfaces, select the option Custom Interfaces and add interfaces to the Allowed Interfaces box. Click Apply . 
 
 Seems like it is only the WAN Interface object. So you should be able to add the Alias Interface there,.