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>3.4. User Settings and Data</H1
><P
> There are other situations where you may want to support
settings and data for each user at site. For instance, if you
scan incoming mail with SpamAssassin (see <A
HREF="datachecks.html#spamscanners"
>Spam Scanners</A
>), you may want to allow for individual
spam thresholds, acceptable languages and character sets, and
Bayesian training/data.
</P
><P
> A sticking point is that SMTP-time filtering of incoming mail is
done at the system level, before mail is being delivered to a
particular user, and as such, does not lend itself too well to
individual preferences. A single message may have several
recipients; and unlike the case with <A
HREF="forwardedmail.html"
>Forwarded Mail</A
>, using an aggregate of each
recipient's preferences is not a good option. Consider a
scenario where you have users from different linguistic
backgrounds.
</P
><P
> As it turns out, though, there is a modification to this truth.
The trick is to limit the number of recipients in incoming
messages to one, so that the message can be analyzed in
accordance with the settings and data that belongs to the
corresponding user.
</P
><P
> To do this, you would accept the first <B
CLASS="command"
>RCPT
TO:</B
>, then issue a SMTP <B
CLASS="command"
>451</B
> (defer)
response to subsequent commands. If the caller is a
well-behaved MTA, it will know how to interpret this response,
and try later. (If it is confused, then, well, it is probably a
sender from which you don't want to receive mail in the first
place).
</P
><P
> Obviously, this is a hack. Every mail sent to several users at
your site will be slowed down by 30 minutes or more per
recipient. Especially in corporate environments, where it is
common to see e-mail discussions involving several people on the
inside and several others on the outside, and where timelines of
mail deliveries are essential, this is probably not a good
solution at all.
</P
><P
> Another issue that mainly pertains to corporate enterprises and
other large sites is that incoming mail is often forwarded to
internal machines for delivery, and that recipients don't
normally have accounts on the mail exchanger. It may still be
possible to support user-specific settings and data in these
situations (e.g. via database lookups or LDAP queries), but you
may also want to consider whether it's worth the effort.
</P
><P
> That said, if you are on a small site, and where you are not
afraid of delayed deliveries, this may be an acceptable way
to allow each user to fine tune their filtering criteria.
</P
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