Configure postfix to use Sendgrid as default Relay
If you would like to configure postfix to use Sendgrid as the mail relay, you can do so by editing the postfix main configuration file.
vi /etc/postfix/main.cf
And then paste the content below at the end of the file :
smtp_sasl_auth_enable = yes smtp_sasl_password_maps = static:username:password smtp_sasl_security_options = noanonymous smtp_tls_security_level = encrypt header_size_limit = 4096000 relayhost = [smtp.sendgrid.net]:587