Restricting Use of Email

If it is necessary to restrict mail through a relay, the checkcompat routine can be modified. This routine is called for every recipient address. It returns an exit status indicating the status of the message. The status accepts the address, queues the message for a later try, and other values (commonly reject the message. It is up to checkcompat to print an error message (using usrerr) if the message is rejected. For example, checkcompat could read:

checkcompat(to, e)
register ADDRESS *to;
register ENVELOPE *e;
register STAB *s;

s = stab("private", ST_MAILER, ST_FIND);
if (s != NULL && e->e_from.q_mailer != LocalMailer &&
to->q_mailer == s->s_mailer)
usrerr("No private net mail allowed through this machine");
if (MsgSize > 50000 && bitnset(M_LOCALMAILER, to->q_mailer))
usrerr("Message too large for non-local delivery");
e->e_flags |= EF_NORETURN;
return (EX_OK);

This would reject messages greater than 50000 bytes unless they were local. The EF_NORETURN flag can be set in e->e_flags to suppress the return of the actual body of the message in the error return. The actual use of this routine is highly dependent on the implementation, and use should be limited.

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Last modified: Friday April 02, 1999.