Error or bounce message similar to the following:
[email protected] Domain xxxx.com has exceeded the max defers and failures per hour (5/5 (100%)) allowed. Message discarded.
This problem can be generated when the email/hosting account is sending emails to many wrong or non-existent addresses, this can be considered spam and the sending of email from the domain is blocked for a period of 1 hour for reasons of security.
In general, there are several circumstances in which this will occur:
1. You are sending mass emails and some of the addresses on your mailing list are inactive.
2. You are sending attachments with a virus.
3. Your account has been hacked and is being used to send spam by a malicious user.
4. You are using forms on your website that are not protected or secured. You can protect your forms for example through a captcha system: https://www.google.com/recaptcha
To avoid this you should:
Verify that the account is not sending spam or unsolicited mass emails.
Verify that you have not been infected by any malicious PHP script in your hosting account
Verify the general bounces of the account that are received in the general webmail of the account, you can access that webmail with your main cpanel username and password by entering webmail.yourdomain.com
If you're going to send mass emails, chances are many of your contacts are no longer using the email addresses they signed up with you. You will need to clean up your mailing list and stop sending emails to these inactive accounts.
If you have autoresponders or forwarders activated in your cpanel, activate spamassasin from the same cpanel and configure it for automatic deletion of emails. This way, if you receive spam with non-existent reply addresses, you will not be forwarding this email to the non-existent box with the autoresponder or forwarder.
Use only your own distribution lists. The people to whom you send the emails must have authorized the receipt of commercial emails and must have the possibility of automatic unsubscription from the distribution list through a link in the email.
You can see all the emails that leave your service through your cPanel - Track Delivery.
Source and original cPanel documentation: https://docs.cpanel.net/knowledge-base/email/how-to-prevent-spam-with-mail-limiting-features/?_ga=2.19177464.1712335749.1688679991-946293897.1688396406#failed- and-deferred-outgoing-message-options
