I set up the queue according to the wonderful blogpost from Joey Keller.
The problem was that sending immediately via email “Sent” button, the limit of 14 emails per second of my SES account, was too slow. Mautic was pushing to many emails to the SES and finally Mautic came up with the message, that no email, or only a few were successfully sent.
After setting up the cron job for the queue:
bin/console messenger:consume email --time-limit=50
it worked, and all 900 emails went out.
One question remaining is: Why did sending to a segment with only 4 contacts also dropped two contacts and sent only to two contacts successfully?