ResolutionThe line in mail headers looks like:
Received: from domain-sender... by domain-recipient...
When Qmail receives a message it does reverse lookup for the both IP which connection is established from and to. So, please make sure that IP addresses are resolved to expected domains at your server.
If you see that an IP address is resolved to several domains, like:
# host 192.168.50.53 localhost
Using domain server:
126.96.36.199.in-addr.arpa domain name pointer alex.org.
188.8.131.52.in-addr.arpa domain name pointer toto.com.
184.108.40.206.in-addr.arpa domain name pointer plesktest.com.
It means that all these domains have PTR record for the IP you are trying to resolve.
It is much more correct to have a single PTR for every IP address.
For example if you need only 'myhost.plesk.com' be always listed in mail headers,
you need to remove all the PTR records for all domains through Plesk CP and leave only one that points to 'myhost.plesk.com'.
You will also need to remove PTR record from Server > DNS zone template to prevent records adding anew during new domains creation.