Flushing infected mail traffic from Postfix server
Postfix consists of a combination of server programs that run in the background, and client programs that are invoked by user programs or by system administrators.
The Postfix core consists of several dozen server programs that run in the background, each handling one specific aspect of email delivery. Examples are the SMTP server, the scheduler, the address rewriter, and the local delivery server. For damage-control purposes, most server programs run with fixed reduced privileges, and terminate voluntarily after processing a limited number of requests. To conserve system resources, most server programs terminate when they become idle.
Client programs run outside the Postfix core. They interact with Postfix server programs through mail delivery instructions in the user's ~/.forward file, and through small "gate" programs to submit mail or to request queue status information.
Other programs provide administrative support to start or stop Postfix, query status information, manipulate the queue, or to examine or update its configuration files.
If Postfix cannot deliver a message to a recipient it is placed in the deferred queue. The queue manager will scan the deferred queue to see it if can place mail back into the active queue. How often this scan occurs is determined by the queue_run_delay. Postfix will scan the incoming queue at the same time as the deferred queue just to make sure that one does not take all the resources and so each can continue to move messages.
The real question is, What is causing messages to be deferred? One of the major reasons that messages are deferred is that your server is going to place mail to “unknown recipients” into the deferred queue if they do not have a legitimate user to go to.
First thing that should be done to analyze the mails that are stuck in the queue is typing the mailq command. If you see a lot of mails in the queue shown in the output, than something fishy is going on on you server. Just looking on the mails, IT people should recognize the domain that has the most mails in the queue. When you find out the domain example.com than the next step to do is to run a bash script that will delete only those mails that are from the infected domain.
#!/bin/bash
match="$1"
find /var/spool/postfix/deferred/*/ -type f -exec grep -l $match '{}' \; | xargs -n1 basename | xargs -n1 postsuper -d
find /var/spool/postfix/active/ -type f -exec grep -l $match '{}' \; | xargs -n1 basename | xargs -n1 postsuper -d
This simple scripts are using bash language to find the deferred and active mails from the user keyboard input on the CLI. After that the script is executing the postsuper -d command that is flushing the queue with that specific domain.
match="$1" is a simple regex that matched text by the first capturing group, in our case a user inputed domain.
After this the mail queue should be emptied with the infected mails and the server will have some freed up resources. Another faster or simple solution, if the mails are not important at the moment, is to flush the complete queue in the deferred folder.
For this we have a simple command: postsuper -d ALL deferred
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