So my smallish (1500+ users) has had on premises exchange servers since the beginning. this year we wanted to look at Office 365/Exchange online. so my boss went out and bought 50 licenses so we can test it out. as I am the system admin he handed me the reins and said start testing. after 6 days of phone calls with Microsoft support I finally got a hybrid setup with my on-premises servers.
I start migrating a few test users, all IT users, and migrating to the cloud is going well UNTIL.... I get an email from a helpdesk tech who is a pilot user/tester and he tells me he can no longer send email from the helpdesk mailbox that he has access to. I look and everything seems to look ok on the on-premises shared mailbox. I then remembered I had a shared mailbox too that I hardly ever used. I saw in my outlook that I still had access to it...