If you see this type of message logged in your outbound mail logs, and your clients are unable to get email delivered to their domain, then I may have the s0lution for you.
In our particular scenario, we have a PHP site running WordPress and the email server is located with IPOWER. So the website needs to be able to send email from our data center into the IPOWER data center, but in our case the emails were getting bounced due to this error below. (VERY NON-DESCRIPT!).
Result: delayed, Status: 4.1.1 452 4.1.1 firstname.lastname@example.org requested action aborted: try again later – GL/GL
in your BOUNCE BACK emails, you may find something like this:
X-Mailer: PHPMailer 5.1 (phpmailer.sourceforge.net)
In our case, the recipient domain was hosted at IPOWER web. To solve this issue, we logged into the IPOWER control panel, went to spam settings:
and went to the WHITELIST and whitelisted our email server’s IP address. Saved the changes and re-tried sending the email on the PHP (wordpress) site.