Recently, Google has started imposing additional requirements on bulk email senders who send more than 5000 messages a day to GMAIL accounts.
For most WordPress sites this will be a non-issue since they’re not sending more than 5000 emails a day. Though, from the press this change has received you’d think the sky is falling.
Still, it is very good practice to do what Google is suggesting because it will increase the deliverability of your emails.