Please read this Geary FAQ.
Pasting here for convenience:
To connect to a Microsoft Exchange server (Outlook.com, Office365, or local institution installations), the Exchange installation must have both its IMAP and SMTP “protocol connectors” enabled. These are enabled for Outlook.com and Office365, and hence Geary can connect to those services. There are no plans to support Microsoft’s proprietary mail access protocols going forward. Thus if the IMAP connector is not enabled for your Exchange installation, please get in contact with your Exchange administrators and ask them to enable it.
When configuring an Exchange account in GNOME Online Accounts, IMAP or SMTP configuration information will not be provided to Geary by Online Accounts, and hence an IMAP account will also need to be configured for Geary to be able to access mail for the account.
Note that using Davmail to access mail on an Exchange server is not supported, since Davmail does not implement any modern IMAP extensions, several of which Geary requires. These extensions (such as IMAP IDLE and UIDPLUS) have been supported by most mail servers (including MS Exchange) since the late 2000’s. Unless Davmail implements these extensions, it will continue to be unsupported. Instead of using Davmail, use Exchange’s IMAP support instead - it is substantially better.
I don’t use Exchange and I can’t verify it works. I hope it helps.