Maileater support only plain-text
Sending an email to maileater is depending to the email client.
Detailed explain in this tecdoc
By default some email clients like Microsoft Outlook will in many cases send 2 content types in the same email (plain/text and html usually).
Outlooks working as described works fine, but not all email client do that.
To avoid to convert the content of the email to plain-text, we would expect maileater process html content.