System created new customer: mailer-daemon@localhost

Zammad 3.3 latest, Centos 7, installed from package. We use IMAP connection to mailbox on local Exchange 2013 server.
Today I noticed, what Zammad system created new customer/user: mailer-daemon@localhost (Mail Delivery System)
There is no ticket for this user in the helpdesk. I am unable to set this user as inactive, because Zammad gives error: invalid email.
So, if e-mail is invalid, how system created such user?
And why system created this user, if no case was created?

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