PostFix

Some implementations of PostFix only allow passwords to contain letters and numbers, meaning you may need to ensure your Elastic Email 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.elasticemail.com]:587

Now you need to specify your credentials (optionally, use apikey as a username and an API Key as the password) in the separate file /etc/postfix/sasl_passwd (you’ll likely need to create it):

[smtp.elasticemail.com]:587 yourElasticEmailUsername:yourElasticEmailPassword

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

Troubleshooting

If port 587 is not working for you please try 2525 in your postfix config. You may also need to edit /etc/postfix/master.cf to remove # from

#tlsmgr unix - - n 1000? 1 tlsmgr

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