We've worked with email marketing companies on these types of campaigns, helping to conduct tests on different tactics to define some clear best practices. Here are the key ones we have developed for these email messages:
1) Be clear in the subject line. Email subject lines like, "Verify your subscription continue receiving [XYZ]" or "Your subscription will end soon" tend to work well. "Goodbye" was another subject line that worked remarkably well. These campaigns tend to be targeting subscribers who have not responded in a while, so breaking the mold with concise, straightforward, or even provocative subject lines help get people to open the email.
2) Restate your value proposition. This is a simple reminder of what your email program offers. A concise restatement of what your subscribers can expect reminds them of what you are all about -- and what they will miss if they do not confirm their email subscription.
3) Use YES and NO options. This is huge! The misconception is that if you only offer a "Yes, please subscribe me" option, that more people will react positively by opting into the program. We have tested using a single "Yes" option vs. the "Yes and No" options several times. In every instance the "Yes and No" option resulted in significantly more opt-ins! There is something about seeing both options that drives more people to respond. Maybe these emails seem like less of a gimmick, more genuine, or more serious. Whatever the reason, the reality is that by including that NO option, you will actually get more people to click YES.
In addition, this approach to email opt-in campaigns provides you with clear answers. There are three resulting groups: 1) Those that opt-in, 2) those that opt-out, and 3) those that did not respond (even if they opened the email). This third group of non-responders will be your target for a second request.
4) Send a second request. There are situations where a second request is not appropriate -- for example, if you are trying to clean a list suspected of containing spam-trap addresses. However, if you have used the YES and NO options cited above, the non-responders are a prime target for a second request for email permission. We find that these second requests consistently get nearly the same number of opt-ins as the first, so failing to do so could have a material impact on the success of your campaign. We have worked with organizations that have tried a third request using the same logic, but the drop-off has been substantial. In our experience, two invitations seem to be the right number.
By following this approach, we have been able to significantly cut down on the number of inactive addresses on several clients email lists. More importantly, we have been able to do so with minimal impact on bottom-line results. A common objection to cleaning a list through a re-opt-in campaign is that dormant customers who "might buy at some point in the future" will be lost. While a valid concern, we simply have not seen this in real experience. In no instance have we seen a perceptible drop in revenue following from these programs -- definitely not one that was more than the amount saved through decreased email volume. You can proceed with confidence!