530 Must issue a STARTTLS command error when sending e-mail using a smart host

Symptoms

When sending an e-mail using a smart host, you may receive the following error:

530 Must issue a STARTTLS command first

Description 

This error message indicates that your smart host requires you to issue the starttls command before authenticating. The hosts_avoid_tls configuration option in Exim causes this.

Workaround

Edit your Exim transport and remove the hosts_avoid_tls option using the Exim Advanced Configuration Editor.

You can find the documentation for the Exim Configuration Manager below:

Exim Configuration Manager