I just had a prospective client reply to a newsletter we sent them in 2010. That’s four years ago, folks. And I bet you have some old emails in your inbox waiting for your attention too, emails you haven’t deleted because you plan to get to them—someday. The oldest email in my inbox right now is three years old. I purge emails on a regular basis, yet I haven’t moved that one to the trash yet. Obviously, I still have every intention of opening it and dealing with it, just not any time soon.

Ditto for promotional emails and newsletters: Just because I don’t open and read them right away doesn’t mean I won’t. I have folders and filters for things I want to read during downtime, and I set those emails aside until I get around to them.

I’m not unusual in my ways. Rather, I’m a typical email user. For the marketer, sending an email might be instantaneous but that doesn’t mean our reaction to it is, which raises the question: How do you really measure the ROI of email? Most metrics are based on looking at email reporting soon after a send, but does that give you the real picture of an email’s performance? Just because a recipient doesn’t open and respond to an email within a set number of days or hours does not mean that message had no value.

There are several ways an email can indirectly influence ROI:

• As already mentioned, an email can sit in an inbox or archive for a very long time until the recipient is ready to interact with it. (Note: That this is another reason for giving lots of attention to your subject line. Making the subject line descriptive and compelling can increase the longevity of an email inbox because the subscriber can remember the reason for keeping such an old email at a glance.)

• Email marketing can influence brand and even customer loyalty without being directly tied to other. A subscriber might not respond to an email but that doesn’t mean the favorable feelings toward your brand weren’t more firmly entrenched in that person’s mind.

• An email can drive an unintended action, such as a purchase at a brick-and-mortar store that’s not traceable back to a message.

You might say old emails never die. They just don’t make it into your email reporting.

When you’re considering the ROI of your email marketing, keep in mind this infographic showing the ROI of your mom. Like moms, email can give us many benefits we might not even recognize let alone measure. But that doesn’t mean we don’t do it. It just means we need to allow for a little wiggle room when we want to quantify that ROI.

About the Author: Scott Hardigree is the Founder of Email Industries. Connect him everywhere, here.