If you define TCPIP$SMTP_FROM without an SMTP domain string (the
    @domain portion of the address), SMTP appends the substitute
    domain name to the text you define. If you do not define a
    substitute domain name, the host name is used.

    For example, the host is configured with a substitute domain name
    of x.com, and the TCPIP$SMTP_FROM logical is defined as follows:

    $ DEFINE TCPIP$SMTP_FROM "bill.smith"

    In this case, the resulting address is as follows:

    From: bill.smith@x.com

    However, if the host is not configured with a substitute domain
    and the host name is host.x.com, SMTP_FROM is defined as follows:

    $ DEFINE TCPIP$SMTP_FROM "bill smith"

    In this case, the resulting address is as follows:

    From: bill.smith@host.x.com