Postfix mail transfer agent¶

Installation¶

Postfix is an open source Mail Transfer Agent (MTA). It is the standard MTA that comes installed on CentOS servers, and one used by the Plesk control panel. If Postfix is not installed on your server, this can be done with the following:

yum install postfix

and then started with:

service postfix start

You can set this to start when the server comes online with the following command:

chkconfig postfix on

Ubuntu installation¶

The process for installing Postfix on Ubuntu is similar. Use the below commands to achieve this:

apt install postfix

Select ‘Internet Site’ on the Postfix configuration screen. Then, the following options:

Mail name: this should be something like mail.domain.com to help with delivery - if you leave this as the default hostname you’ll have issues with bounced mail.

Root and postmaster mail recipient: The username of your primary Linux account.

Other destinations to accept mail for: $myhostname, example.com, mail.domain.com, localhost.domain.com, localhost

Force synchronous updates on mail queue?: No

Local networks: 127.0.0.0/8 [::ffff:127.0.0.0]/104 [::1]/128

Mailbox size limit: 0

Local address extension character: +

Internet protocols to use: all

Start and enable the service in one go with:

systemctl enable --now postfix.service

The server is now ready to send emails. You can configure your websites to connect to localhost over TCP port 25 and they will not require any authentication.

Please ensure that you have a valid hostname and rDNS set. See this guide for details

Where are the logs?¶

Postfix logs are generally found in /var/log/maillog on Plesk servers. The log will show a variety of different information including mail sent, received and authentication attempts. The logging can be modified by making changes to the Postfix configuration file found in /etc/postfix/main.cf. There are a variety of different configuration changes that can be made and it would be best to refer to the official Postfix documentation for a detailed description.

Queue Manipulation¶

You can use the below commands via SSH to help in debugging mail issues. To view the mail queue in Postfix you will need to use the below command.

postqueue -p

This will display all mails in the queue including their ID, the sender and recipient address, the time and the reason that it is in the queue. Using the ID, you can view the contents of the email and the headers by using the below command where [ID] is the ID of the email.

postcat -q [ID]

This will show a lot of information regarding the email and will help in investigating possible mail compromises such as whether the mail was sent through a logged-in account, which would indicate a mail account has been compromised, or through a PHP script which would indicate a malicious script has been uploaded to your server.

Clearing the Postfix mail queue¶

You can use two methods to clear the queue. The first method is to ‘flush’ the queue, or attempt to send mail in the queue. This is not guaranteed to clear the mail as it will attempt to send all of the mail in the queue. You would use this method if there is lot of mail in the queue due to a network issue or a blocked port, and the issue has been resolved.

postfix -f

The second method would be to delete the mails in the queue. You would do this in an event such as the server was found to be sending spam mail and the queue is full of the spam mail. Once you have stopped the spam from sending then you would want to delete the contents of the queue. To do this, you can use the following commands.

To delete a specific mail from the queue you would use the below command where [ID] is the ID of the mail you wish to delete.

postsuper -d [ID]

If the spam is only associated with a single domain, you can use this script to search for and delete only mail associated with that domain, reducing the possibility of accidentally removing legitimate mail. Replace @example.com with your domain.

mailq | tail -n +2 | awk 'BEGIN { RS = "" } /@example.com/ { print $1 }' | tr -d '*!' | postsuper -d -

To delete all mail in the mail queue you would use the below command.

postsuper -d ALL

The server is now ready to send emails. You can configure your websites to connect to localhost over TCP port 25 and they will not require any authentication.

Please ensure that you have a valid hostname and rDNS set. See this guide for details

Where are the logs?¶

Postfix logs are generally found in /var/log/maillog on Plesk servers. The log will show a variety of different information including mail sent, received and authentication attempts. The logging can be modified by making changes to the Postfix configuration file found in /etc/postfix/main.cf. There are a variety of different configuration changes that can be made and it would be best to refer to the official Postfix documentation for a detailed description.

Queue Manipulation¶

You can use the below commands via SSH to help in debugging mail issues. To view the mail queue in Postfix you will need to use the below command.

postqueue -p

This will display all mails in the queue including their ID, the sender and recipient address, the time and the reason that it is in the queue. Using the ID, you can view the contents of the email and the headers by using the below command where [ID] is the ID of the email.

postcat -q [ID]

This will show a lot of information regarding the email and will help in investigating possible mail compromises such as whether the mail was sent through a logged-in account, which would indicate a mail account has been compromised, or through a PHP script which would indicate a malicious script has been uploaded to your server.

Clearing the Postfix mail queue¶

You can use two methods to clear the queue. The first method is to ‘flush’ the queue, or attempt to send mail in the queue. This is not guaranteed to clear the mail as it will attempt to send all of the mail in the queue. You would use this method if there is lot of mail in the queue due to a network issue or a blocked port, and the issue has been resolved.

postfix -f

The second method would be to delete the mails in the queue. You would do this in an event such as the server was found to be sending spam mail and the queue is full of the spam mail. Once you have stopped the spam from sending then you would want to delete the contents of the queue. To do this, you can use the following commands.

To delete a specific mail from the queue you would use the below command where [ID] is the ID of the mail you wish to delete.

postsuper -d [ID]

If the spam is only associated with a single domain, you can use this script to search for and delete only mail associated with that domain, reducing the possibility of accidentally removing legitimate mail. Replace @example.com with your domain.

mailq | tail -n +2 | awk 'BEGIN { RS = "" } /@example.com/ { print $1 }' | tr -d '*!' | postsuper -d -

To delete all mail in the mail queue you would use the below command.

postsuper -d ALL