interface
Use: smtp
Type: string list, expanded
Default: unset
This option specifies which interface to bind to when making an outgoing SMTP call. The variables $host and $host_address refer to the host to which a connection is about to be made during the expansion of the string. Forced expansion failure, or an empty string result causes the option to be ignored. Otherwise, after expansion, the string must be a list of IP addresses, colon-separated by default, but the separator can be changed in the usual way. For example:
interface = <; 192.168.123.123 ; 3ffe:ffff:836f::fe86:a061
The first interface of the correct type (IPv4 or IPv6) is used for the outgoing connection. If none of them are the correct type, the option is ignored. If interface is not set, or is ignored, the system's IP functions choose which interface to use if the host has more than one.