After sending the letter, the remote server returns an error:

450 4.7.1 Client host rejected: cannot find your hostname, [89.xxx.187.xx]

The reason is incorrectly specified settings or mismatched records

MX - PTR - Hostname.

To determine which hostname is specified in the server settings, run the following command:

1. Determine the hostname of our mail server

telnet 89.xxx.187.xx 25

The server will greet you and inform you of its hostname
Trying 89.xxx.187.xx...
Connected to mail.synay.net.
Escape character is '^]'.
220 mail.synay.net ESMTP Postfix

We have defined, in our case, this:

mail.synay.net

2. MX record must match hostname

Let's check the DNS records of our domain, there should be an MX record that matches the record in the hostname

3. Check the PTR record

nslookup.exe 89.xxx.187.xx

The DNS server should return a reverse record of the form:
mail.synay.net
Most likely it will be different for you and this is the main reason for the error.

Contact your hosting or Internet provider to specify a return record (PTR) for the IP address of your server.