How to limit exim4 processes for outgoing emails

If you want to do this as is, you are going to need to write a custom exim ACL that extracts the sending domain.

Alternatively you can run php via su_php or use mod_ruid2 to run each vhost as a unique user. Or block direct sendmail injection and require all scripts to send email via authenticated SMTP accounts. Then you can use all of the existing available ACLs.

Remove the * from dc_relay domains. This allows anyone to relay. If you are relaying for your local network put its IP-range(s) in dc_relay_nets.

If you want to allow authenticated users to send mail from anywhere you need an accept rule for authenticated users. The default configuration should include the appropriate rules to bypass checks for authenticated users so that they can send email as if they were on the local network.

If you add AUTH_SERVER_ALLOW_NOTLS_PASSWORDS to your /etc/exim4/exim4.conf.localmacros, you will be able to test authentication on an non-secure connection. It is best practice not to allow this as anyone who can sniff the network can obtain your credentials.

You may want to add this macro to enable the submission port for remote users. The submission port is the preferred port for remote submission.

 daemon_smtp_ports = 25 : 587

In my default configuration the server-side authenticators are commented out. I have used them successfully for plain text and md5 authentication. Userid and password are the second and third fields.