mail to unknown user in AIX 3.1

[Guy Harris]

>Has anyone an idea why I would get the following message when I 
>tried mailing to an unknown user:
>
>   ----- Transcript of session follows -----
>550 xuser... User unknown: Permission denied
>
>Permission denied for what?

Nothing, necessarily.  The "Transcript of session" thingie looks as if
it comes from "sendmail", and "sendmail" has been known to be truly
stupid when it comes to composing error messages; the routine that
composes them tends, if "errno" is non-zero, to tack on the error
message for the value of "errno" *even if the error that set "errno"
occurred a while back and has nothing whatsoever to do with the error
that it's reporting*.

Some later versions of "sendmail" will zero out "errno" in some places
to keep this from happening; perhaps AIX 3.1's "sendmail" isn't recent
enough, or perhaps even the later versions missed some cases.