Have you configured one of more of your email clients to use POP?

If you are missing emails or finding that emails are not syncing across your devices, it may be that one or more of your email clients (applications) is configured to use POP.

For each email client check the settings to ensure they are configured to use IMAP. IMAP allows emails to sync and update across multiple devices.  

Please refer to the article: Modify mail server settings - Outlook

Keep email offline setting may need to be adjusted

Some emails may not be downloaded to Outlook as they are outside of the time range configured in Outlook. Please refer to the article: Not all emails are syncing to Outlook

Not all folders are subscribed

On each device where you are using Outlook, ensure that you have subscribed to all folders. 

Refer to the article: Folders are missing / Error synchronising subscribed folders

Other Outlook support issue

If you require additional support for Outlook, we recommend reviewing the support resources provided directly from Microsoft:  https://support.microsoft.com/en-au/outlook