There isn’t an out of box way to format the emails that come out of SharePoint by default; they do say it’s coming though. In the meantime, there is a work-around. It’s not an automated solution, but it’s better than nothing if you absolutely cannot use the default mail that comes out :
Here’s how to do an email digest in case you don’t know how.
Add a News web part to a page. Make sure there are more news articles available than are displaying on the page or you won’t see the See All link.
Then click Email News Digest.
Select the news you want to send out and Next.
Put in a “format” heading and send it to yourself.
When you get the email digest in your inbox, forward it, then while it’s in edit mode, change whatever you need to – but leave the articles info alone so it doesn’t break any links by accident, then send it on to your audience. Don’t forget to change the subject line too.
Done.
Views from Veronique 
[…] is still in the “promise” phase for any formatting options, but Veronique Palmer has found a way to format it from the Outlook Desktop app – now that is neat! I tried it with Outlook-on-the-web and you can do a few things there as […]
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how we can manage when we have a multipe langage site, and we want each user receive his news digest in the right langage?
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could it be edited in Power Automate?
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That is a royal pain! Log a call with Microsoft and make them explain it to you.
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I have been doing this for more than a year. All of a sudden last week, Outlook 365 will strip the formatting from the digest email when I attempt to “forward” (so I can apply my own formatting before sending out). It looks awful, and is a pain to reformat for distribution.
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