Symptoms
When you try and open an email from the Webmail interface you could receive an error message saying "Could not load message from server"
Description
The reported behavior is caused by invalid MIME types in the header information that are not recognized by the IMAP or dovecot services. In some cases, this can be caused by message data corruption but is most commonly caused by the headers generated on the remote mail server.
It's important to note this is an issue caused by the sender of the message and not the cPanel or Webmail services.
Workaround
While it is possible to adjust the Dovecot configuration on the system to handle these types of message, the best solution would be to contact the sender of the message and make them aware of the issue so they can fix the problem on their end.