You can't, unless you downgrade to v68 and stick to that version.

Ok, many thanks Stans...

which alternative Email programs would you recommend, apart from Outlook?

Thunderbird 68.

Hmm, not sure if that is such a good idea, since in a few years, security issues will be extremely large with this version, right? Plus, changes in the way emails work will not get addressed in this old version, so I think eventually it just won't work any more, right?

Well, some users are willing to trade security for looks, so they'd rather use old versions regardless of their unpatched holes. Nope, it's NOT a good idea IF you care about security holes. The new look quite frankly isn't so hard to get used to. Users mindset is the only obstacle. In your case, I don't see how harder it is to check your multiple recipients. All that has changed, is scrolling horizontally instead of vertically.

see https://support.mozilla.org/en-US/kb/new-thunderbird-78#w_new-addressing-area

For me, security AND other new capabilities trumps old functionality.

Perhaps you usually only send to one or two recipients?

Have you ever tried to manually compare two really long lists in two windows? Two lists, with the target list being a list with multiple entries in one, loooooong line, in which you are only able to see a handful of entries, and which you need to scroll horizontally by scrollbar every few seconds in order to see a another handful of entries? With the source list being a top to bottom list with one entry per line, which needs to be scrolled vertically every 7.4 times you have scrolled horizontally in the other list? Has it helped you to keep your one index finger on a list entry in the horizontal list, and the other on a list entry in the vertical list? Did you enjoy losing track of where you are in the vertical list, whenever you needed the index finger to operate the mouse, in order to scroll further on? I've had to do that like a million times before, and every time when I had to do it, I was cursing even louder.

Microsoft has long back decided that "users want it that way", and therefore now many windows require me to scroll horizontally to read and compare a single, extremely long line. That's more than "just a mindset" - it's one of the main reasons to use Linux instead of Windows, whenever I can.

If the delevopers want to destroy usability even more, here's another great tip from Microsoft: Make everything in such a way that it cannot be highlighted with the mouse and copied with CTRL-C, so that the user cannot simply cheat himself around the problem by copy/pasting the long line into a text file, inserting newline characters, and then compare line by line!

Sorry to have to say this, but I want security and usability.

Whatever you say. My lists are curated long before I start writing a message. I use the address book manager for that. By the time I'm hitting write, all I have to do is add the list to whichever field I want, but hey, to each his own.