Why is E-mail not being delivered because the recipient's address doesn't exist?

Question

Email

We received an email from a specific address on our gmail. The sender's address on a public e-mail service. When trying to reply to this email, the server is notified of an error 550 : User unknown , so the message does not go away. When viewing the technical information about the incoming letter, it was found that the address is correct. Actually, the question is6 what can cause this (besides the fact that the person immediately after sending the letter demolished the box, which is almost impossible, since it concerns work) and how to fix it? We need to send some very important information. and quickly, that's why I'm asking the question.

Answer 1

550 : User unknown replies MTA
MTA configures admin
and antivirus
and default settings
to whether or not the user himself demolished his account this is distantly related
specifically to your case - ordinary antispam ("don't call here again")

Answer 2

Because the administrator of this server wanted it that way. Notification postal codes are generally such a strange thing that no one bothers to use the way they want. It is not at all necessary that such a user does not exist, perhaps on this server they do not want to accept mail coming to this address, but such a code was put in order to get rid of it faster.
If you have a working postal contact or phone, it's easier to contact in this way, because you don't have any means of influence :)

Answer 3

dimonchik2013 , in the end, turned out to be right. After a little investigation and reading a Reddit thread, it turned out that ezweb.ne.jp cuts letters from PC_devices (only accepts mobile devices) as spam, unless otherwise configured.