Microsoft is adding support for custom email addressing to Office 365 email services, a feature it hopes to complete in Q3 2020.
Custom email addresses are an optional feature that some email providers can support. The feature is described in the RFC 5233 internet standard.
Officially known as subaddressing, this standard allows users to extend their email address using "tags" or the plus (+) character, hence its two alternative names of tagged addressing or plus addressing.
For example, a user with the email address of firstname.lastname@example.org can use the plus addressing feature to extend their email address to email@example.com.
If the user's email address supports subaddressing, all emails sent to the firstname.lastname@example.org email will land in the user's email@example.com inbox.
The "tag" can be unique, allowing users to generate an infinite amount of custom email addresses, which they can give out to companies in newsletter sign-up forms, contact forms, and other online forms.
Once emails arrive in the user's inbox, the owner can see which email address the sender used.
If the email arrived from a company called "Home Flower Delivery" but used the email firstname.lastname@example.org, the email owner can easily determine that Netflix (in this case) has shared his email with another company.
However, despite its benefits in helping users track how their email addresses get shared online, plus addressing is not widely supported across email providers.
Google has supported plus addressing in Gmail since its early days, but Yahoo, AOL, and most others did not.
Microsoft added support for plus addressing to Hotmail in 2013; however, the feature was never supported with its Exchange package part of the Office 365 commercial suite.
In a feature roadmap page, Microsoft says plus addressing is currently in development, and hopes to have the feature live for all paying Office 365 customers by the end of Q3 2020.