The Fix Video/oEmbed Links AddOn will “undo” the formatting that many email clients add to links which will enable videos or any embeddable content supported by WordPress to be displayed inline rather than as a link. This will only work for services supported by WordPress or added by another plugin. See https://wordpress.org/support/article/embeds/ for a list.
For example, you paste https://www.youtube.com/watch?v=VMT9fQz60XY into your email. By the time Postie sees the email it looks like <a href=”https://www.youtube.com/watch?v=VMT9fQz60XY”>https://www.youtube.com/watch?v=VMT9fQz60XY</a> which WordPress doesn’t know is a link to a video so it is just displayed as a link.
With the addon enabled Postie will recognize the content as a YouTube video and alter the markup so WordPress knows how to display it correctly.