Lightning mail-mailbox settings-stationery and signature, and then set the format of new mail and forwarding reply, that's it.

Web format is generally used. If it was originally in web format, switch to text first, and then change to web format.

Text: There is no hyperlink, that is, no button is clicked on the page, and all text becomes text.

Web page: the title of the front and source files is gray, and the text is black. This format is also used normally.

Original file format: it seems nothing special.