For the HTML detection,it should skip for base64 images,such as:

img src="data:image/png;base64,iVBORw0......"

as the image can be very large and takes long time to handle.

Email body is parsed in two phases:

1. Pre-processed phase - In this phase, words are searched in the original email message as-is.
2. Post-processed phase - The server formats the message by removing any noise words such as HTML tags, comments, and any character encoding.

If you are planning to search for HTML tags or any word that you want to parse as-is, check this box.