sending mail is not a standard POP feature, and it's not what sendmail
uses.
Your choices for limiting access to sendmail include:
1. Limiting the addresses it listens to. You don't want it listening
to public IP addresses.
2. Using /etc/hosts.{allow,deny} to control what addresses sendmail
accepts connexions from.
3. Using an external firewall to control who can connect to your mail
server. This is appropriate, for example, when you use ADSL and have a
"hardware" router manage your internet connexion. You can also choose
to use a PC in this role (I do it with an HP Vectra Pentium II running
Debian and Shorewall).
4. Using netfilter on your mail server as above. See www.netfilter.org
and "man iptables."
5. Sendmail (probably) has its own additional means of controlling who
can connect: I use Postfix, and for certain and sure Postfix has.
I'd like to add another idea to the list. You can use DRAC (Dynamic
Relay Authorization Control) to dynamically update your SMTP relay
rules. We used to use this at one of my previous employers. You're users
must make a POP or IMAP email check, and then DRAC will dynamically add
a rule to your relay access filters for them.
http://mail.cc.umanitoba.ca/drac/
-Shawn
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