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Changing the Postfix maximum email size

The configuration option is "message_size_limit" and by default is set to 10240000 bytes which is roughly 10MB. You can see what setting your postfix install is currently using by issuing the following command:

postconf | grep message_size_limit

Just doing "postconf" on its own will show all the options so using grep on "message_size_limit" filters the output to just the setting we want. For the default setting, you'll see this:

message_size_limit = 10240000

To change the setting to some other value, open the main.cf file in your favourite text editor (it's often at a location like /etc/postfix/main.cf) and either edit or add the following line, depending if it's already in the file or not.

message_size_limit = 20480000

In the above example I've changed it to ~20MB.

Then reload postfix like so:

service postfix reload

and your setting will take effect. You can query postconf again if you want to check all is as expected.