With that message opened up in a window by itself, go to the menu and select the following:
View » Message » All Headers
Or simply press
SHIFT-CMD-H
From here, you should be able to discern what the originating server may have been. For example, here are some sample headers I have for an email from Amazon:
Code:
Return-Path: <[email protected]>
Return-Path: <[email protected]>
X-Original-Messageid: <urn.correios.msg.xxxxxxxxxxxxxxx@xxxxxxxxxx.rte-svc-onebox-na-1a-i-11a5c06d.us-east-1.amazon.com>
Note that I replaced long strings of numbers with the "x" character instead.
There are various others that reference
mac.com and my
.me email address, which indicate who the recipient was and the name of the receiving email server. The examples listed above should indicate what servers the email originated from, and thus clue you in as to who sent them. If it came from your email provider to the email address you have with that provider, then the domains should mostly be identical, barring cases like Apple where they have multiple domains available to the same user (.mac, .me, .icloud).