Mail sent with HTML body is converted to plain text

Dear all:

I'm using FM SO_NEW_DOCUMENT_SEND_API1 with document_type = 'HTM', and filling object_content with an internal table of type solisti1 with HTML content.

When I check the sent mail in transaction SOST the content is correctly displayed, but when it si sent, I receive it converted to plain text, that is, showing the HTML tags instead of formatting it properly.

I've been looking for information, but no succeed.

Could it be a basis configuration problem?

Regards

Jordi