I embedded in a custom autoresponder message a YouTube video with the "Insert/Edit Embedded Media" icon from the content editor. It seems to work in preview mode of the message in the Console, but in the message that is actually emailed to my clients the video is either replaced with a black square or is absent from the message. What am I doing wrong?
There is nothing wrong in what you did or in the Console. Most email providers forbid the embedding and playing of videos inside emails. So, even if you embed a video in the message created in the Console, it will not appear as such in the email that is sent your contacts. You can instead insert a link to the video in the message, using the direct link (URL) to the video, so that the recipient can click on it an be taken outside the email box and watch it on the external website that hosts the video, like on the YouTube website.