Today I got an absolutely ridiculous problem, after noticing that I've not been receiving emails at all, since the morning...

I decided to do a test, by sending an email to my mailbox, from an external address, and here is the result:

550 5.7.1 Service unavailable; client [xxx.xxx.xxx.xxx] blocked using zen.spamhaus.org

After investigating, I see the problem is similar to #1781

And gladly, someone's comment drove me to the solution, which is to overwrite the following configs in "postfix-main.cf":

postscreen_dnsbl_sites
smtpd_recipient_restrictions

Removing the offender instructions for zen.spamhaus.org.

Now, I have a few questions:

1. What does it mean that emails are "blocked using zen.spamhaus.org" ?
   Does this mean my domain/IP was blocked somehow?
   Is this something that only affects me, or nothing to do with "me" ("my domain") ?

2. Why are these "sites" in my configurations? Who decides to put these sites as default?

I'm just very concerned with the fact that these 3rd party sites, that I never heard about before, can so easily "break" the receiving of emails on my server...

3. Is there a way to recover all the emails that were lost during the day?

This is just absolutely ridiculous...

I'm not blaming you at all. I understand you don't control "zen.spamhaus.org".
But I definitely want to prevent this from happening in the future.

I'm on the latest 7.x version.

Thank you.