When sending an e-mail using a smart host, you may receive the following error:
530 Must issue a STARTTLS command first
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.
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: