Outbound NAT Choices

  • Hello. I am looking for some advice on selecting between two configurations for an outbound NAT situation. I have an email server with an internal IP address of
    Currently it uses a dedicated VIP (CARP) of X.X.X.27 when receiving email (and other email related connections). This inbound connection is set up as a port forward:

    If      Proto    Ext. port range    NAT IP                        Int. port range
    WAN      TCP      25 (SMTP) (ext.: X.X.X.27)    25 (SMTP)

    Unfortunately, my email server is still set up to use the interface IP address of X.X.X.26 for outbound connections (i.e. sending email). This gets me into some trouble with email servers that compare the HELO address (.26) to the DNS resolved address (.27).

    I noticed that I may have two choices for configuring the outbound connections to use X.X.X.27. Please tell me which you think would be the better way between these two:

    Manual Outbound NAT Rule:

    Interface  Source              SrcPrt  Dest    DestPrt  NATAddress    NATPort  StaticPort
    WAN      *      *          *        X.X.X.27      *          NO

    1:1 NAT:

    Interface          External IP          Internal IP
    WAN                X.X.X.27/32

    Things to note:
    X.X.X.27 will be dedicated to the email server IP address and no other internal IP address will share it.
    I am already using manual outbound NAT mainly to map my primary LAN ( to the default interface IP address (X.X.X.26) and also to map my secondary LAN ( to a VIP (PARP) IP address (X.X.X.30). This email server falls within the primary LAN. I am oblivious to the difference between a PARP and a CARP address and where I can and cannot use each type.

    Thanks for any input.


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