Without any announcement, the number of additional addresses one could configure for an Office 365 mailbox has been capped to 200. The issue was first reported on the Answers boards here: http://answers.microsoft.com/en-us/msoffice/forum/msoffice_o365admin-mso_manage/new-limit-on-proxy-addresses-unable-to-delete-old/12b5c44f-c2ed-408f-9dda-fd45b76ba820.
If you try to add any aliases over the 200 limit, you will get the following error message now:
While this definitely not something that major (200 is still *a lot*), pretty much every enterprise customer I’ve worked with so far has had a need to mass provision aliases to work with some 3rd party tools. They can still continue doing so, but will have to accommodate the solutions to use multiple mailboxes instead.
I’m yet to discover an article that details the new limit or gives some explanation as to why it was changed. The “Exchange Online limits” doesn’t mention anything about aliases, and the only other reference is actually listing the limit as 100: https://support.office.com/en-us/article/Add-additional-email-aliases-to-a-user-in-Office-365-0b0bd900-68b1-4bf5-808b-5d240a7739f4?ui=en-US&rs=en-US&ad=US.
I’ll update the article with more info when such is available.
UPDATE: The limit has now been increased to 400 and KB article has been posted on the issue: https://support.microsoft.com/en-us/kb/3196424.