From domain Option

The  SMS   option, , specifies the domain name to append to SMS source addresses when constructing envelope From addresses for e-mail messages. The name specified should be the correct name for routing e-mail back to SMS, (for example, the host name associated with the MTA SMS channel). If not specified, then the official host name of the channel specified with the    SMS   option (CHANNEL in legacy configuration) will be used.

See also:
 * mta_channel Option
 * official_host_name Option
 * SMS gateway_profile options