Need to think about setting quota to mail users in case i open my mail server for friends or further business needs, say web hosting customers. Suddenly, i found out that the buddled MTA for CentOS, sendmail uses mbox format, and save incoming mail in /var/spool/mail instead of /home/. But the users customized mailboxes are saved in /home//mail such that setting quota for individual mail users is not feasible. Also, as convinced by Dan Bernstein, i agreed that using mbox mailbox format is not as reliable as using maildir mailbox format.
Regarding reliability, i recalled that when i was serving Sun at the 1st yr., i.e. 1997-1998, there were frequent mailbox corruptions. Why? At that time, no support staff gave me an acceptable answer. Maybe i was not technical enough. Now i guess we were using an unreliable MTA (Mail-Transfer-Agent). Also, using NFS mounting the home directory requires extra reliability of MTA.
Anyway, i'm convinced that by common sense that every mail is saved as a separate file in maildir format is more reliable that saving every mail as appending to the mailbox file consisting of numerous mails especially when you think of having network connection interruption as saving a mail such that the whole file would be corrupted and then even a resend or rewrite could not fix the problem.
By using qmail, i am more acquainted with the 3 major components of mailing system: MTA, MDA and MUA. They must be working in harmony so as to receive incoming mails through SMTP by MTA, deliver to appropriate users by MDA and can be retrieved by mail client through MUA.
Also, i started to be awared that spammers can abuse open relays of SMTP servers to spread out unsolicited mails. However, not to open SMTP server or exactly relaying all mails to the SMTP server from the ISP is a "responsible" act.
One last important piece of note here is that the last line of the run script located at /var/qmail/supervise/qmail-smtpd/ has to be modified as below to make it work: