We have a user mailbox on Office 365 that we share out to a department at our company with around 20 users. We've had some problems with the mailbox regularly updating for some users.
Utilizing a Shared Mailbox is not an option due to the size and required functionality of the mailbox (sending out emails via TLS through O365).
We're being told by our Vendor of Record that provides support (as a middleman) for Office 365 that you cannot have that many users connected to a user mailbox with Office 365 but I cannot locate any documentation to support such a claim.
Thanks in advance for any assistance.