To send a message to all users on linux system, you can use wall command, which sends a message to everybody logged in with their mesg permission set to yes. The message can be given as an argument to wall, or it can be sent to wall’s standard input. When using the standard input from a terminal, the message should be terminated with the EOF key (usually Control-D).

Examples

To send a message “Alert ! Web server under DOS attack !”, type the following command

$ wall
Alert ! Web server under DOS attack !

When the message content is complete, press Control-D (CTRL+D) to send message to all users.

To display message “Alert ! Web server under DOS attack !” to all admin group member use wall command with “–g” option as follows:

$ wall -g admin
Alert ! Web server under DOS attack !

When the message content is complete, press Control-D (CTRL+D) to send message to all users.

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Check pending mail

To check how many pending mail, type the following command

# mailq

This will give you example output like this:

 /var/spool/mqueue (2 requests)
-----Q-ID----- --Size-- -----Q-Time----- ------------Sender/Recipient-----------
p4BHPCuP002174      105 Thu May 12 00:25 mualaco
                 (host map: lookup (nixonbox.com): deferred)
                                         pedromelon@nixonbox.com
p4B8uNPZ023735     1000 Wed May 11 15:56 saigonlab
                 (host map: lookup (gmail.com.vn): deferred)
                                         tranhai.ht.83@gmail.com.vn
                Total requests: 2

There are several techniques to delete sendmail queue

Manually removed pending mails

Type the following commands

# rm -rf /var/spool/mqueue
# rm -rf /var/spool/clientmqueue

Final, restart mail service with the following command

# /etc/init.d/sendmail restart

Output like this

Shutting down sm-client:                                   [  OK  ]
Shutting down sendmail:                                    [  OK  ]
Starting sendmail:                                         [  OK  ]
Starting sm-client:                                        [  OK  ]

You can also use find command to remove “Deferred: Connection refused” queues

# find /var/spool/mqueue -type f -exec grep "Deferred: Connection refused" {} \; -exec rm -rf {} \;

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How do you managing the mailbox size for users configured with the Postfix mail server ?. And how to avoid “error writing message: File too large” ?

Setup new mailbox size limit

Login as root and type the following command to edit main.cf

vi /etc/postfix/main.cf

Default, the limit for any local mailbox or maildir is 50 MB but, this can easily be changed with by using this directive in the main.cf file.

mailbox_size_limit=102400000

This changes the mailbox limit to 100 MB. Or you can make it umlimited by using a “0”.

mailbox_size_limit=0

Setup new message size limit

Default message size is 1 MB, example changes the message size limit to 2MB. Open file /etc/postfix/main.cf and add/modify/set values as follows

message_size_limit = 2048000

Review the current settings

If you would like to review the current settings on your Postfix server use this command

postconf -d | grep size_limit

Sample outputs

body_checks_size_limit = 51200
bounce_size_limit = 50000
header_size_limit = 102400
mailbox_size_limit = 102400000
message_size_limit = 2048000

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