What can I do when my subscriber can't find the confirmation message?

If a subscriber reports a missing confirmation message to you, there are two approaches you can take based on the time frame of the subscription. 

Less Than 12 Hours After Their Subscription

Because often times the issue with missing confirmation messages is attributed to ISPs take a little longer than normal to put messages into the inbox and because sending additional confirmation message makes the link in the first invalid, there is a 12 hour period where no additional confirmation message can be sent.

For the meantime, you might suggest to the subscriber that they:

Look for the message in any other folders it may have been mis-routed to

Wait an hour, then check for it again

Use a different email address

If after this period of time, the message still is not found, they may need to take additional measures.

More Than 12 Hours After Their Subscription

If the message is still not found after this period, your subscriber should:

Return to the sign up form they originally filled out and enter their information once more to have another copy of the confirmation message sent

Contact their ISP and check any filters on their own system that may be affecting the deliverability of the message

Although most issues with the delivery of the confirmation message can be resolved through working with the postmaster of most email providers, the quickest and most straight-forward way to a solution is simply to have the subscriber use another address that likely will not carry the same issues.

If you haven't yet suggested this resolution, you might strongly consider doing so at this point.