>We are writing a gateway too and it would help us be a
>good network citizen if we knew what LISTSERV will
>interpret as a rejection notice.
 
I know listserv checks the subject field as follows:
 
 If subj = 'UNDELIVERED MAIL' |,         /* Crosswell MAILER */
    Left(subj,22) = 'UNABLE TO DELIVER MAIL' |,   /* Mult*cs */
    subj = 'RETURNED NETWORK MAIL' Then  /* VAX-type  MAILER */
    Call RejMail 'mail subject indicates a delivery problem'
 
I believe he also checks to see if the sender was a "defined
trusted mailer" from, ultimately, the BITEARN NODES file.
/mrg