Sending invalid emails succeeds when debug.redirectAllMailTo is set
debug.redirectAllMailTo is set, mails work without error even if they are sent to invalid mail addresses.
This makes spotting problems in development harder. It's probably hard to avoid this, but it might be useful to highlight this downside in the documentation.
(#7013 prevented the sending of emails in our latest feature in production. Since the feature sends several identical emails we never noticed the mails with invalid to adresses in development - and it took a while to realize the bug really was present there as well.)