I just realized that usermin denies access when trying to send mail outside its domain. I can send mail to another person on the same domain but I cannot send mail to gmail or hotmail for example.
Was this by design? I have tried changing some configuration in Servers - > Postfix -> SMTP Authentication And Encryption.
I can send emails no problems with a webmail like roundcube, it is just webmin that doesn't work and I assumed it didn't auth and maybe expected local relaying but having check marks for that in SMTP Authentication And Encryption did not seem to make a difference.
I have two copies of virtualmin running with webmin and they both provide this error.
Any ideas?
Have a look in your /etc/postfix/main.cf file. What have you got for smtp_relay_restrictions and mynetworks?
in the main.cf file I ended up blocking this
smtpd_relay_restrictions = permit_sasl_authenticated reject_unauth_destination permit_inet_interfaces permit_mynetworksrestart postfix and now I can use webmin to send email from the domains I host.
I didn't add this line nor did any modifications so I can only assume that this was created on a fresh install of CentOS7 or virtualmin install. I will spin up a VM to see for myself.