P1 and P2 Headers in SMTP

P1 = the value on the MAIL FROM command of the SMTP connection (the message envelope) as defined in RFC 821.
P2 = the email address in the message body as defined in RFC 822. These include the FROM, REPLY TO and SENDER fields

For example, the following SMTP command sequence describes where P1 and P2 are used:

HELO server
MAIL FROM this_is@my_p1_address.com
RCPT TO: recipient@domain.com
DATA
FROM: this_is@my_p2_address.com
TO: recipient@domain.com
SUBJECT: This is a blog on P1 and P2

This is the text of the message
.

The MAIL FROM value should be your email address, but it does not have to be (ie one of the reasons why spam is so prevalent)
The FROM: header should match this, but this value is what is displayed in the email in Outlook (and other clients). The P1 address is used for routing and not display.

If the connection to an Exchange Server is anonymous then the P2 address will contain the display name and the email address, but if it is an authenticated connection then the P2 email address will be resolved to the value in the address book and this value will be displayed.