Email Send button doesn't do anything

Trying to send email to a contact from inside Dynamics 365. The Send button doesn't do anything. All I can do with the email is save it as a draft. This is the case for every use who has tried it.

We're using the Microsoft Exchange Online server profile and Server-Side Synchronization for Incoming, Outgoing, and Appointments. The "View Unapproved Mailboxes" screen is blank. All mailboxes are showing Incoming, Outgoing, and Appointments Status as "Success." Incoming emails are being correctly tracked to appropriate accounts. A custom workflow successfully sends emails to users.

Any ideas?

Thank you,

Matt