You should look at the full headers. To do that, go to File > Info > Properties while the email is selected.
The following information may help you understand the details about email headers.
(Source: here)
What is an email header?
The email header is the information
that travels with every email,
containing details about the sender,
route and receiver. It is like a
flight ticket: it can tell you who
booked it (who sent the email), the
departure information (when the email
was sent), the route (from where it
was sent and how did it arrive to you)
and arrival details (who is the
receiver and when it was received). As
when you would book a flight ticket
with a false identity, the same goes
for emails: the sender can partially
fake these details, pretending that
the email was sent from a different
account (common practice for spammers
or viruses).
How to interpret email headers?
Starting from the assumption that you
want to read an email header because
you want to know who really sent it,
let's take an example (we will ignore
the header tags that do not give
precise information about the sender).
The following email was received by
support@emailaddressmanager.com and we
want to see who the sender is. Here is
the email header of the message:
As you may already noticed, there are
three paragraphs starting with the
Received tag: each of them was added
to the email header by email servers,
as the email travelled from the sender
to the receiver. Since our goal is to
see who sent it, we only care about
the last one (the blue lines).
By reading the Receving From tag, we
can notice that the email was sent via
corporate2.fx.ro, which is the ISP
domain of the sender, using the IP
193.231.208.28. The email was sent using SMTP ("with ESMTP id") from the
mail server called mail.fx.ro.
Looking further into the message, you
will see the tag called
X-Originating-IP: this tag normally
gives the real IP address of the
sender. The X-Mailer tag says what
email client was used to send the
email (on our case, the email was sent
using FX Webmail).
