550-Please turn on SMTP Authentication in your mail client

Symptoms

You receive the following error when sending or receiving mail:

550-Please turn on SMTP Authentication in your mail client

Solution

You can consult the following resources for more information on troubleshooting this issue:

https://forums.cpanel.net/resources/troubleshooting-550-please-turn-on-smtp-authentication-in-your-mail-client.565/

New or transferred account unable to receive mail: Rejected relay attempt | 550 Please turn on SMTP Authentication | $HOST is not permitted to relay through this server without authentication