The conventional way for a mail client program to send e-mail is using TCP port 25, which is also the port used by mail servers to talk to each other. But port 25 is now widely abused by malware to spread worms and spam. As a result, many ISPs are restricting its use

You want to log email Subject field into /var/log/maillog?
By default the email subject field is not logged to the maillog file. To log it, here is the trick.

SMTP Authentication is a scheme which was introduced in 1999 by J. Myers of Netscape Communications and finally released as RFC 2554 (“SMTP Service Extension for Authentication”). It is partly based on the SMTP Service Extensions as defined in RFC 1869.

If an email message is undeliverable, it will bounce back with ‘MAILER-DAEMON’  telling you that the message could not be delivered to the destination addresses. The queue will try to keep delivering based on your mail server configuration. Unfortunately it will make your mail server takes some resources to the queues. And we dont want this thing happened.