Mysterious Email problem

I just had a wierd problem with people/friends/clients (let's just call them clients):

Client #1 has Google suite for emails with multiple native domains. But mostly only one domain is used for email.

Client #2 has a personal Gmail account. He also has his business email (with a non-gmail domain) as a receive and sender email added to his personal GMAIL.
For his business email he uses the native SMTP server of the business domain's webhost to send.

Client #1 has not noticed any problems with emails whatsoever. Same with Client #2 - he regularly sends and receives email on both his personal and his business email without issue.

However yesterday Client #2 sent an invoice to #1 from his business address. The email never arrived.
No delivery failure notification was received. It just failed silently. No the email is not in #1' junk/ads/spam.

If #2 sends #1 an email from his personal address from the same Gmail dashboard it arrives without issue.

If #2 puts additional to #1 in to TO another receiver into BCC or CC they get the email, just not #1.

If #1 sends an email to #2' personal address it works.

The first really valueable error message is: if #1 sends #2' business address an email they get a "failed delivery" response that reads:

Die Adresse wurde nicht gefunden
Ihre Nachricht wurde nicht an #2 zugestellt, weil die Adresse nicht gefunden wurde oder keine E-Mails empfangen kann.
Die Antwort vom Remoteserver ist:
-- 550 Verification failed for #1 Unrouteable address Sender verify failed

I am not sure yet who is at fault. No obvious email DNS errors. But it's quite interresting. To be continued.

Update 2020-01-20 to 2020-01-27:

So I have solved this now. Both #1 and #2 use the same webhost. My fault was not turning off the email server for #1. When #2 got the email from #1 it was looking it up on the webhosters' email server instead of going through DNS, and when it didn't find the sender address there, #2 denied receiving #1' email.
In the other direction #1 never got emails from #2 because again the sender didn't go through #1' DNS but trying to deliver to the unused webhosts' email server.

What is the moral of the story? Always test from webhoster server to webhoster server, and from gmail to gmail, and from and to a different third party. You never want local delivery to mask an error with DNS. And you never want a correct DNS mask an error with the local configuration.