Your recipients’ behavior is one of the factors that many ISPs (Gmail, Yahoo!, HotMail, etc.) use to determine whether or not your messages should be delivered to the Inbox or to the Spam folder.
One of the ways to get a stamp of approval from recipients is to ask them to add your FROM address to their address book (or their “Safe Senders” list), which is easily done in most email clients. In fact, many senders now explicitly ask recipients to do just that, even before an email is ever sent to them (e.g. “Thanks for signing up for our newsletter. To ensure that our messages are not mistakenly put in the Junk folder, please add <some_email_address> to your contacts.“).
However, most messages we have seen forget to indicate that there is another, really important way for recipients to help legitimate senders avoid getting mistakenly delivered to the Junk folder. It’s the “No Spam” button. That is, if your messages are – for whatever reason – delivered to the Spam folder, your recipients can help you deliver future messages to the Inbox by flagging them as “Not Spam”.
Note that this can positively affect your deliverability not just into that specific recipient’s inbox, but to all inboxes. Although the exact formula is obviously not shared, we know that ISPs are factoring user engagement into the process of determining whether a message may be spam: multiple recipients clicking on the “Not Spam” button clearly sends a strong signal about the reputation of that specific sender.
Here is the “Not Spam” button shown in Gmail…
… and here is the Yahoo! Mail one:
In your future messages, consider explicitly asking recipients to use the “Not Spam” button in the unfortunate circumstance in which a perfectly good message ends up in the Junk folder.
For example: “If you find this message in your Junk or Spam folder, please click on the Not Spam button to tell the system that this is not spam. Thank you!”
Since user engagement matters more and more, it makes sense to try to engage your recipients in this way too!