why rewrite From header?

[Tim Myers]

Our web server uses DNS CNAME entries for its virtual hosts. Messages sent
from those virtual hosts are having their From: headers rewritten using the
A record hostname.

CNAME records are not IP spoofing, why are they being rewritten?

How can this be turned off?

Tim Myers
RHEL AS 3.0
MailScanner-4.29.7-1

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