If you have several email addresses, it's easy to forget which one you used when you subscribed. More importantly, even if you only have one email address, you may still find that the newsletter is using a variation of this address, but one that still reaches you. If you've kept the welcome or confirmation message you may have received when you signed up, you might find the email listed within. Obviously you can also look at an issue of a newsletter to find out which email address they're using. Then you have to look at the so-called email headers to find out the email address used by the newsletter.
Every email contains the actual message plus the headers.
Most of it, though, is hidden from view. After all, most of the header information is uninteresting to the non-specialist and would simply clutter your screen. Of course, you may be wondering how you can see this header information if it's hidden from view. This depends on the software you're using. This brings up a window where you can see all that email's headers.