Using CiviCRM 4.7.14 on WordPress 4.7.3. The configuration uses the default CiviCRM mail() mailer with a WordPress Gmail SMTP plugin.
Everything worked fine until about a week ago, but no configurations were changed. The Gmail SMTP Plugin sends mail successfully, but Civi does not. Any help on where to look would be greatly appreciated.
The error is:
Oops. Your MAIL settings are incorrect. No test mail has been sent. An error occurred when CiviCRM attempted to send an email (via SMTP). If you received this error after submitting on online contribution or event registration - the transaction was completed, but we were unable to send the email receipt. The mail library returned the following error message: mail() returned failure This is probably related to a problem in your Outbound Email Settings (Administer CiviCRM » System Settings » Outbound Email), OR the FROM email address specifically configured for your contribution page or event. Possible causes are: Your Sendmail path is incorrect. Your Sendmail argument is incorrect. The FROM Email Address configured for this feature may not be a valid sender based on your email service provider rules.