I noticed that your "post" has gone unanswered. In the unlike event you are still monitoring this forum or for someone else who may read it in the future, I will take a shot at it. You have asked a very complex question for which there is no easy, simple answer. The first thing you need to realize is that the privacy of this specific e-mail is dependent upon the security of the sender's system, the e-mail process and the security of your system.
The e-mail process is the easiest so I will address it first. The ONLY effective solution is END-TO-END ENCRYPTION! The e-mail including any attachments must be encrypted on the sender's system BEFORE it is saved & transmitted and only decrypted on your system when you want to read it or use any attachments. There is no other way. You may or may not know but most e-mail is LEGALLY scanned by private companies who build profiles on your which they sell to anyone willing to pay. You probably do not recall but you agreed to this in the EULA (End User License Agreement) when you, and the sender, setup your e-mail accounts!
I don't mean to get too technical but there are two types of e-mail: TEXT and HTML. The war is over and HTML has won because it is what companies and users want to use despite the rantings of some technical people. So, you, and the sender, need to use an e-mail program that supports HTML and what is called MIME so any attachments also are encrypted. They are actually not difficult to use but they are probably beyond the abilities of the average user to setup. Google & Yahoo are working on an implementation of end-to-end encryption and have said that it will be available sometime in 2015. Personally, I use Thunderbird with the Enigmail plugin and have PGP/MIME enabled for attachments and people who only have a TEXT ONLY e-mail programs (pictures are encrypted and sent as attachments).
Now, for the security of the sender's and your systems. You asked a specific question but the answer is, "It is completely & totally dependent upon the implementation of the e-mail program!" Unless and until all programs, including e-mail programs, address the privacy & security issues, all you can do is take steps to minimize and reduce the exposure of confidential information. And, even then, ALL systems and programs "leak"! By that I mean, no matter how carefully built and implemented, information can be exposed "accidentally" through bugs, oversight and the like. So, you cannot depend upon one, single solution but have to take multiple steps to minimize exposure. Using Eraser is only one step toward securing your, and your sender's, systems!
My first question to you would be, "Do you use a complex, secure password?" If not, there is no reason to do anything else because your system cannot be secured! Secondly, are you willing to invest the time, money and effort to do more to learn what to do, how to do them, buy any necessary hardware & software and then actually implement them on your systems? For example, each release of Windows is more secure than its predecessor. Windows 7 is more secure than Windows Vista and I hope you are not still using Windows XP as it is out of support. Also, Windows Professional can be made more secure than the Windows Home editions. How important is it for you to really secure your privacy?
As for these specific things you can do, I will list a few things but this list is by no means complete and I intentional left off Eraser functions:
1) Use a complex, secure password
2) Encrypt your pagefile and do a one-time "clear" of the pagefile (but don't clear it at every shutdown)
3) Remove the hiberfile as it cannot be encrypted (yet) "powercfg -h off"
4) Encrypt C:/User/[User Id]/AppData because most programs store their data in folders & files under here!
5) Encrypt ALL confidential data files
6) Encrypt ALL your Backups
7) Encrypt the Drives/Partitions using BitLocker or 3-party software
One warning, you will lose all of your data if you "lose" your encryption certification or forget your passphrase so make certain you understand what your are doing and store them both in multiple safe locations!