I don't know how or why, but I missed it.
Chris warned about it way ahead of time, and I still missed it.
So what is "it"? "It" is the blacklist relays.ordb.org, and it was announced a long time ago, that they were closing down. So some e-mails didn't get to my server. Since I am using a private whitelist, it wasn't immediately obvious that e-mail was bouncing, since some e-mails got through.
As Chris puts it:
This is simply a reminder to lazy or incompetent administrators to manage their use of DNSBLs actively. You can't just define once a list of DNSBL zones you want to use for email blocking and then leave it unmaintained.
Now, does this make me lazy or incompetent? Lazy, I think.
Anyway, now it's fixed, and e-mail is flowing as it should again. If anyone has been sending e-mail to me during the last couple of days, and got a bounce message back, please try again.