Important news about Shared Mailboxes in Exchange Online

Microsoft is announcing some technical changes and enforcements to Shared Mailboxes in Exchange Online. See, "Correcting Shared Mailbox provisioning and sizing" on the Exchange Team blog.

tldr; "Do as I say, not as I do."

For a long time the documentation has stated that Shared Mailboxes over 50GB in size require an Exchange Online license, but in reality customers have been able to create Shared Mailboxes up to 100GB. The 50GB limit has not been enforced until now. In order to reduce issues associated with this technical change, existing Shared Mailboxes in EXO are grandfathered in, but will come under these new limits if they are reconfigured. The blog does a good job of explaining when and
how the enforce limit will apply in several scenarios. I encourage you to read it.

The bottom line is that Shared Mailboxes under 50GB still do not require a license. Shared Mailboxes over 50GB require an Exchange Online P2 license.

All this highlights an important point - you should always design and support your IT policies and infrastructure based on the documentation, not what the actual behavior is. Microsoft can choose to implement limits and technical blocks at any time to match the written documentation. You should never be out of compliance.