Graymail is not something new, but I had never heard of it. Graymail is about emails that form a gray area between wanted and unwanted messages. It's an interesting and handy concept for the reader, and certainly for the sender of emails.
Graymail is not spam
The meaning of spam is probably known by everyone. Spam is not in a gray area. Unsolicited emails that bombard you with vague promises and offers. Most email software providers already know how to deal with it quite well, and this scourge seems to be under control.
Graymail is bulk email that does not fit the definition of spam because it is solicited, comes from a legitimate source, and has varying value to different recipients.
Graymail is email that you open sometimes
Graymail have in common with spam that they are sent in bulk. But these e-mails form a gray area between wanted and unwanted. You've signed up for one thing or another at some point, consciously and often unconsciously. You ask for a free e-book and with this you get the marketing emails or newsletter of a company or creator. But is also caused to receiving what feels like a never ending stream of messages. Think of LinkedIn updates, security messages, Instagram messages, notifications on delivery status, terms and conditions updates etc. etc.
Nothing wrong with that per se, it's an exchange in the attention economy, and you might want to dip into the stream of information at some point in time again. But the engagement on those emails is very low, and they can clutter up your inbox.
Google's Gmail, by the way, tries to address Graymail by applying filters for it. As a result, they don't show up in your inbox right away.
Graymail as a useful concept
So Graymail is a nice term for the recipient who needs to quickly make a (mental) triage of the dozens of emails that are in the inbox every day. But it is also a useful term for senders. Is the email to my relations relevant, does it offer value, and will it lead to engagement? The key measure of this is engagement. Is the email opened, does it lead to an action.
Apple's Mail Privacy Protection and graymail
Apple, the largest provider of email clients, has started to shield interactions on emails by what they called Mail Privacy Protection (MPP). With this protection enabled, no longer information about the reader goes out when opening an email. About 50% of all emails are opened with an Apple client and so this change has caused quite a stir among email marketers. The privacy measures have made it a lot harder to say for sure whether readers are opening or interacting with a received message.
In that sense, the gray area for senders has just become a lot more gray.