This is just some notes on things I do in Postfix.

1. look for messages that are sending to multiple recipients

   grep 'postfix/smtp' /var/log/mail.log | cut -d ':' -f4 | grep -v 'connect' | grep -v warning | grep -v SSL | sort | uniq -c | sort -nr | head

2. Grab one of the identifiers and look at where it originated. This returned 47B5F826C2

   grep 47B5F826C2 /var/log/mail.log | grep 'from=' | cut -d':' -f5 | cut -d'<' -f2 | cut -d'>' -f1

3. Choose a username. This returned mail.brakzijn.nl@my.server

   grep mail.brakzijn.nl mail.log | grep RCPT | cut -d'[' -f3 | cut -d']' -f1 | sort | uniq -c | sort -rn | head

4. see the IP they are coming from, then look for the login that is associated with it

   grep 129.205.113.219 /var/log/mail.log | grep 'sasl_method=LOGIN' | cut -d'=' -f4 | sort | uniq -c | sort -rn

5. That is the user that has been hacked

The mail log (/var/log/mail.log on Debian derivatives) contains way too much information most of the time. Here, grep is your friend. You can find different things based on which daemon is reporting the line item, then further refine by adding an additional grep after.

For example, if you were moving a domain from one server to another, it is common to allow the old server to accept messages for a while to keep from losing your clients e-mail. So, for example, if you wanted to know the last time any mail was delivered to an example.com account, you would do.

grep delivered mail.log | grep example.com | tail

• If you want to see who is sending mail, try looking for the submission daemon in the log. You can then narrow it down with a second grep

  grep submission /var/log/mail.log | grep keyword_to_search_for

• If you want to see who is receiving mail, look for the delivered keyword in the log. This only shows you mail that has been accepted and delivered.

  grep delivered mail.log | grep keywork_to_search_for

Actually, this is the total message size. It is in bytes, so it is a very large number.

To see what the current setting is, type

postconf | grep message_size_limit

. If the value is not explicitly entered in main.cf, the default value is show.

To set a new value, type

postconf -e message_size_limit=26214400

. The number is 25 Megabytes, or 25*1024*1024 (from a calculator). Obviously, you need to reload or restart postfix for this to take effect.