When the mail that is the HTML format is sent to the maileater, the mail body will be ignored. It is necessary that the plain-text mail is sent for the email generation. This behavior is a design.
Page 429 in the Administration guide explains as follows.
"Note: The Mail Eater does not support emails in the RTF or HTML-only formats."
And page 426 in the Administration guide explains as follows.
"Incoming messages that are sent HTML-only, without a plain-text version included, lose their message body. If the message matches any mailbox rule filters with an empty message body, a ticket can be created with an empty Description, or with the quoted message subject as the entirety of the ticket description."