This is so simple to do, but I have to look it up every time I need it (not something that comes up regularly!);

When configuring a development server, you may find you have a need to ensure that emails will not be sent to any domain except those you explicitly permit (for example if you’re using real-world data to do some testing, do you want to send all those users irrelevant emails?).

This documentation details how to configure Postfix on a Linux server to disregard any mail sent to domains that are not explicitly permitted.

Don’t use IPTables

You could, of course, add two IPTables rules to the outgoing chain. The first of which would allow connections on Port 25 to the domain you wish to allow, the second blocking connection to any server on Port 25.

It’ll block the mail from being sent, but will mean that every one of those messages sits in the mail queue for 60 days until it’s disregarded. You could reconfigure the timeout, but given the ease of the steps below, what’s the point?

Use Transport Mapping

Using this method, we can tell Postfix to either reject the mail, or disregard it. The latter is generally the preferred method as we want the sending application to believe the mail has been sent. Continue reading “Configuring Postfix to block outgoing mail to all but one domain” »

Postfix by default restrict attachment size to approx 10MB i.e. 10240000 bytes.

You can check it using following command:

postconf | grep message_size_limit

To change attachment-size to say 50 MB, run a command like:

postconf -e message_size_limit=52428800

Note:
If you are running a mail-server with SMTP/IMAP access, you need to change postfix attachment size only. I spent half-hour debugging dovecot to increase attachment size, just to realize that above change in postfix config was all I needed!