This behavior is possible if the sending mail product does not provide this information in the P2 (Body) of the message. This behavior can be reproduced in the following manner.
Note <CRLF> represents carriage-return/line-feed or the use of the ENTER key.
telnet tcp/ip address of mail host 25
helo
mail from: sender@example.microsoft.com
rcpt to: recipient@example.microsoft.com
data
<crlf>
.
<crlf>
For the To, From, and Subject fields to be populated, the sending mailer must populate that information in the data section. The behavior described in the "Symptoms" section does not break any RFC; however, it is not a recommended, preferred, nor appreciated behavior for a mail system to engage in.