Since the problem exists in both Outlook and Mail, this points to your data and setup.
We've had several complaints about this on the Microsoft Outlook forum, but not everyone has this issue. You need to determine where the problem is located.
1) Is it your data?
2) Is it your network?
Are you fully updated to 14.2.3? Have you started over in a new Identity?
How to move your data to a new Identity
Have IT setup a test account for you. Testing Outlook on a new Exchange account would quickly eliminate your Exchange folder setup. If it's quick and no CPU issues then look at your folders. However if it goes wild with no data, then talk to IT.
If you can select not to sync folders and other data (it could be an old calendar event with attachments) this would eliminate corruption in older data.
The Inbox syncs more frequently than any other folder and should be kept as clean as possible. (this is the number one cause of problems with sync). Do NOT make subfolders under the Inbox.
- Outlook has trouble with some network connections.
- Some have found Spotlight indexing caused their issue. You might need to re-index your drive.
- If you don't have enough free disk space, this can cause odd issues with Outlook. You need a minimum of 10% free.
- I've even seen issues where a draft message was trying to upload the server causing this issue.
Never rebuild in Identity with an Exchange account. Delete the Exchange and any IMAP account(s) THEN rebuild. Add accounts back after rebuilding. Better yet, start over in a new identity. I would never rebuild as long as an Identity will open. I export my data and import into a new Identity.