Add exchange server public IP and the domain name into o365 allowed list.
Please refer below for more details,
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/create-safe-sender-lists-in-office-365?view=o365-worldwide
Further you can check and control SCL level. Please refer following for further details,
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/spam-confidence-levels?view=o365-worldwide
https://learn.microsoft.com/en-us/microsoft-365/security/office-365-security/use-mail-flow-rules-to-set-the-spam-confidence-level-scl-in-messages?view=o365-worldwide
https://learn.microsoft.com/en-us/exchange/antispam-and-antimalware/antispam-protection/scl?view=exchserver-2019
Other tip is to check Domain or IP black list status. Please check the Domain/IP blacklist status which are relevant to you Exchange server.