It might have to do with this so I'll add some of my thoughts.
The setup of the server is iRedMail with Active Directory. So this list is a distribution list created in Active Directory and it's much bigger than the other four. It has more than 2000 users.
Perhaps postfix has a limit for how many users a mail list can have?
Error in mailog:
postfix/pipe[22814]: 6CDC9F800014: to=<maillist3@mydomain.com>, relay=dovecot, delay=0.05, delays=0/0.01/0/0.04, dsn=4.3.0, status=deferred (temporary failure)
Also in queue I get (temporary failure)