The response from the remote server was:
550 Please turn on SMTP Authentication in your mail client. mail-qk1-f172.google.com [18.104.22.168]:36503 is not permitted to relay through this server without authentication.
Manage SMTP restrictions
Click Enable to deny users the ability to bypass your mail server to send mail. To allow users the ability to bypass your mail server, click Disable.
(WHM >> Home >> Security Center >> SMTP Restrictions)