Configure postfix to relay all email via a selected server

Edit your /etc/postfix/main.cf file if you have a line starting with

smtp_tls_security_level = (something)

insert a # at the begining of that line. Then  add the following lines to the end of the file

# Enable auth
smtp_sasl_auth_enable = yes

# Set username and password
smtp_sasl_password_maps = static:YOUR-SMTP-USER-NAME-HERE:YOUR-SMTP-SERVER-PASSWORD-HERE
smtp_sasl_security_options = noanonymous

# Turn on tls encryption 
smtp_tls_security_level = encrypt
header_size_limit = 4096000

# Set external SMTP relay host here IP or hostname accepted along with a port number. 
relayhost = [YOUR-SMTP-SERVER-IP-HERE]:587

then

systemctl restart postfix

Tested on Fedora 40