You need to complete these given prerequisites, you can skip the step if you have already completed.
Sign up for a Mailazy account.
Complete Domain Authentication.
Generate the Mailazy Access Key
Integrate Mailazy with Postfix
Updating the Postfix configuration to use Mailazy as a relay host is easy. For more advanced configuration scenarios, you'll need to refer to the Postfix documentation.
Some implementations of Postfix only allow passwords to contain letters and numbers, meaning you may need to ensure your Mailazy password is only alphanumeric.
Find your Postfix config file, typically /etc/postfix/main.cf, and add the following:
smtp_sasl_auth_enable = yes
smtp_sasl_password_maps = hash:/etc/postfix/sasl_passwd
smtp_sasl_security_options = noanonymous
smtp_sasl_tls_security_options = noanonymous
smtp_tls_security_level = encrypt
header_size_limit = 4096000
relayhost = [smtp.mailazy.com]:587
We highly recommend using an API key as your credential for any service that supports it. API keys are both more secure and easier to work with.
Now you need to specify your credentials (use mailazy_apikey as username and an mailazy_secret as password) in the separate file /etc/postfix/sasl_passwd (you'll likely need to create it):
Next, make sure the file has restricted read and write access only for root, and use the postmap command to update Postfix's hashtables to use this new file:
$ sudo chmod 600 /etc/postfix/sasl_passwd
$ sudo postmap /etc/postfix/sasl_passwd
Finally, restart Postfix:
$ sudo systemctl restart postfix
If you are getting no mechanism available error messages, it generally indicates that you are missing some SASL authentication libraries.
Install the missing module dependency using apt-get (i.e., Debian, Ubuntu):
$ apt-get install libsasl2-modules
Or using a yum (i.e., RedHat, Fedora, CentOS):
$ yum install cyrus-sasl-plain