How to fix Joomla SMTP errors
SMTP connect() failed
Errors like “Joomla SMTP connect failed” are mostly evoked by the wrong content in the “SMTP Security” field. Find the right way to follow it here. If you are using Gmail, turn on account access here and unblock captcha here.
SMTP authorization errors
The following From address failed: email@example.com : MAIL FROM command failed,553,5.7.1 Sender address rejected: not owned by auth user.
Authorization errors are mostly evoked by the wrong username or password. Note, that for some mail servers (like Yandex) you should type the username, not mail address (example, not firstname.lastname@example.org).
Joomla doesn’t send messages without any errors
It may happen with contact form Contactus. SMTP forbid to add foreign emails to the Sender field, so you should turn off these options in the “Modules manager”.