Returns 505 to remote servers

[Mattia]

When external servers authenticate with other mx, 505 error is returned and therefore the connection is denied.
Analyzing the mail the sending ip that reveals the server (posteio 2.3.5 # 1628) shows a local ip mask 192.168.0.1.
The email id appears without '@domain.com' like the others, but as follows: [e-mail].
The emails from this customer do not arrive, they are not present in the admin "quarantine" list.

Enabling the public ip in the white list no connection is seen from the address.

The public address of the remote server is contained in the mail ider.

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