As part of our defense against spam we have in place a limit on the number of deferred or failed emails per hour sent from our mail servers, this limit being 25%. You may be wondering what this means? In essence, if 25%+ of emails sent from your domain within a one hour bounce back, all mail from your account will fail for the rest of that one hour period.
This protection assists where one's account has been compromised and is silently sending spam (which often is very hit and miss resulting in many failures), so you'll be instantly aware something isn't right. And it assists web admins in pruning their mailing lists which may have high numbers of now inactive email addresses.
If you're encountering this issue and you're not sure exactly what is causing this, you can probably determine the cause by using the Email Trace option in your cPanel. Login to cPanel >> click on Email trace. When you click the Email Trace icon, you'll see a field where you can enter a recipient's email address and then click a "Run Report" button to get information about email sent to that recipient.
If you enter nothing for the recipient email address, you'll get back data for all email traffic, and as you look through it you should see groups of bounced messages which can help you determine what sender caused the problem, and why. They will typically be emails sent to email addresses which you've not seen before, often (AOL, QQ.com, yahoo), and the subjects will typically sound like spam.