Some users reported that creating an SPF record solved the issue.
- Login to CPanel > Tweak Settings > All> “Restrict outgoing SMTP to root, Exim, and mailman (FKA SMTP Tweak) and switch it from “On” to “Off”.
- The problem may be the cPHulk (login blocking software). So adding the external IP of the server’s cPHulk blacklist. The solution is to add the IP to the cPHulk whitelist may resolve the problem.
Even if one of the above it’s not your case I can say for sure this is not a plugin issue, you need to contact your mail service support.
If you still want me to check your issue, please contact me on the contact form page.
If you had this issue and you solved it, please comment below.