Mail server scalability

What problems do we come across?

Linear password files

On some systems, every mail delivery and pop3 connection requires a scan through the whole /etc/passwd file.

Linear mbox files

If a user keeps their mail on the server, every POP3 connection requires the POP3 daemon to read the entire mail file

Too many files in one directory

CPU limits

Disk performance

Keep your SMTP (smarthost) and POP3 services separate

Keeping SMTP and POP3 on separate machines makes it much easier to scale your mail service.    -- does not relay, accepts incoming SMTP for
                               delivery to local mailboxes only    -- relays, has no local mailboxes

There is an additional advantage: mail routing works correctly even if one of your customers leaves (moves their domain's MX records to point somewhere else) without telling you.