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Relay mails from Postfix through other SMTP server

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Sun, 07 Feb 2021 15:42:00 -0800

It's common to have the IP #blacklisted if one #self-host. In some cases port 25 is blocked by the #ISP and hosting providers. At other times the big companies like #Google, #Yahoo, #AOL, #Microsoft, etc, may block the IP for various unknown reasons.

Here is a tutorial to relay mails from Postfix through other #SMTP server. I assume that you have working Postfix server.

Note: If one #relay through other server, it not only makes that server to send the mails on ones behalf, but allow the other server to know the #content of the email and can change the content of the #email too. Thus seen as a big compromise.

Edit /etc/postfix/main.cf
Add these line to the main.cf
```
smtp_sasl_auth_enable = yes
smtp_sasl_security_options = noanonymous
smtp_sasl_password_maps = hash:/etc/postfix/saslpasswd
smtp_always_send_ehlo = yes
relayhost = smtp.domain.tld
```
Change smtp.domain.tld with the SMTP providers address and add the SMTP port, if the SMTP port is different for that provider. Eg. relayhost = smtp.your_domain.tld:port
Create a file saslpasswd in /etc/postix. Eg. /etc/postfix/saslpasswd
Add username and password of the SMTP provider in the saslpasswd file. Maintain the format show below.
smtp.domain.tld yourlogin:yourpassword
Hash saslpasswd by running following command.
$ postmap /etc/postfix/saslpasswd
Restart Postfix.
$ service postfix restart

That’s it!
Test it and see Postfix #logs for traces of the mail.

postfix smtp relay mail