Edit the Postfix configuration file /etc/postfix/ using an editor such as Nano. Below, we have provided a sample of what the config should be changed to.

The USERNAME and PASSWORD should be changed to the SocketLabs SMTP username and SMTP password. Both of these credentials are available in the Control Panel of your SocketLabs server, found in the Configuration dropdown of the menu bar under SMTP Credentials.

# sudo nano /etc/postfix/

smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = static:USERNAME:PASSWORD
smtp_sasl_security_options = noanonymous
smtp_tls_security_level = may
header_size_limit = 4096000
relayhost = []:2525
relay_destination_concurrency_limit = 10

The relay_destination_concurrency_limit ensures that Postfix doesn’t try to open too many concurrent SMTP connections.

You will then need to restart Postfix. 

# sudo /etc/init.d/postfix restart