2 non-obvious reasons for “550-Please turn on SMTP Authentication” error in cPanel & Exchange servers, and what to do about them
SMTP error from remote mail server after RCPT TO:<firstname.lastname@example.org>: host domain.com [xx.xx.xx.xx]: 550-Please turn on SMTP Authentication in your mail client. 550-(mail.server.net) [xxx.xxx.xxx.xxx]:36825 is not permitted to relay through 550 this server without authentication.
All popular mail servers have SMTP authentication enabled for security protection. Mail accounts using those servers need to authenticate first, before trying to send mails.
At times, users can try to send mails from their mail clients without enabling authentication. ‘550-Please turn on SMTP Authentication’ is a commonly reported error when users try to send mails without authenticating.