Plus Addressing, also known as subaddressing, is a defined way to support dynamic, disposable email addresses for mailboxes which makes it easier to filter your email.
Instead of signing up for a newsletter with firstname.lastname@example.org for example, you may sign up to a newsletter with email@example.com and then use Outlook rules to easily filter all incoming mail associated with that address.
Plus addressing support has been available in Exchange Online since September 2020 as an opt-in feature, and then could only be activated by a Powershell command.
Today Microsoft announced that the feature will be enabled by default in the future, and asked admins to prepare for this.
The reason for Microsoft’s caution is that Exchange has long supported real (non-disposable) email addresses with + in the name, and these email addresses may stop working when the feature is enabled.
Microsoft is asking admins to check their accounts and remove such email addresses, and are giving admins a long lead time to do this before Plus Addressing becomes active by default.
Currently, new Exchange Online users have Plus Addressing supported by default already, and everyone else will have it enabled by default in January 2022.
Notably, it appears Outlook.com already supports Plus Addressing, so regular users can already create instant disposable addresses, though of course, it would be trivial for spammers to strip the +ACDE bit off to get your real address.
Admins can read more about Plus Addressing at Microsoft here.