The GFI MailEssentials Dashboard always shows the SMTP sender email address which is transmitted during the SMTP transaction and it is usually discarded afterwards. This data can be empty if the email was intentionally composed as this or if the email are NDRs (send from postmaster's email address). If the postmaster email address has never been configured in Microsoft Exchange server, this SMTP sende email address will be blank.
Here an example:
Message Id: <firstname.lastname@example.org>
Scan Date:10/16/2014 10:55:48 AM
Received Date: 10/16/2014 9:51:06 AM
Subject: Undeliverable: GFI MailEssentials Spam Digest
Scan Result: Ok
The sender email address which is seen in the email client (for example: Microsoft Outlook, Thunderbird and so on) when composing or opening an email is the MIME email address which is transmitted after the DATA command during the SMTP transaction.
Please find further information about the SMTP protocol and the difference between SMTP and MIME email address in the link here