As announced at Microsoft Ignite 2020, support for plus addressing is now available in Exchange Online. Thanks to our UserVoice site, this feature was identified as the number 1 request for Exchange ...
Updated Oct 27, 2020
Version 3.0
Blog Post
EDIT: Got some answers on my own since the feature finally started functioning in our org.
I have a few questions:
1) The detail "Support is currently scoped to mailboxes with support for distribution lists and Groups expected in December 2020" is worded oddly, or maybe by brain isn't functioning today. Just to clarify, this means that Plus Addressing currently works with user mailboxes, shared mailboxes, and resource mailboxes? And that support for distribution lists and M365 Groups is *not* ready now, but coming in December?
Tests worked to user/shared/resource mailboxes.
2) Is Plus Addressing applicable to any valid SMTP address, or only primary SMTP addresses? (e.g. if my primary SMTP is andrew123@company.com and I have an additional address of andrew345@company.com, then andrew123+newsletters@company.com should obviously work, but would andrew345+newsletters@company.com also work?)
Works for any aliases on a mailbox in my testing.
3) If a mail security appliance (e.g. Cisco CES) does SMTP Call-Ahead to Exchange Online to validate an addresses' existence before delivery, will Exchange Online correctly validate an address using Plus Addressing since it's not an explicit/"real" address on Exchange Online mailboxes?
External messages using Plus Addressing are flowing through our CES appliance fine, so yes, Exchange Online does respond to SMTP Call-Ahead correctly.
4) How long after enabling Plus Addressing with Powershell should it take to function correctly? I enabled it for my org 2 hours ago, and it is still not even functioning internally. (user mailbox to user mailbox within our own tenant)
It took about 6 hours to finally start working.