Source: http://kb.sp.parallels.com/en/115472

My server is using the Postfix mail server. For some reason, all messages it sends contain unnecessary headers, like the following:

X-No-Relay: not in my network

or

X-No-Auth: unauthenticated sender

The number of headers is multiplied by the number of message recipients, e.g., the headers of a message sent to two recipients would look like this:

X-No-Relay: not in my network
X-No-Relay: not in my network

Cause
Postfix adds those headers from these files:

# ls /var/spool/postfix/plesk/*re
/var/spool/postfix/plesk/non_auth.re
/var/spool/postfix/plesk/no_relay.re

Resolution
The issue will be fixed in a future update for which a new Knowledgebase article will be created. You may subscribe to new articles on Parallels Plesk Panel products if you wish to receive notifications about their creation.

Workaround
If you do not want messages to contain the above headers, clean (empty) these files:

/var/spool/postfix/plesk/non_auth.re
/var/spool/postfix/plesk/no_relay.re

nano /var/spool/postfix/plesk/no_relay.re

/^/ PREPEND X-No-Relay: not in my network