If you view your email using the IMAP email protocol, then over time your inbox will grow larger and larger. This will cause slow email performance, and Outlook may not sync correctly.
Two solutions to this are:
1. Use Outlook's archive feature (classic Outlook only)
Note: Archive and AutoArchive are available in classic Outlook for Windows (Outlook for Microsoft 365, Outlook 2024, 2021, 2019 and 2016). Microsoft says AutoArchive is not available in the new Outlook for Windows. If you use the new Outlook, use option 2 below.
To archive old email once: click File, then Info, then Tools, and select Clean Up Old Items. Choose the folder to archive, select a date under Archive items older than, and click OK.
To archive automatically: click File, then Options, then Advanced. Under AutoArchive, click AutoArchive Settings, then choose how often it runs and how old items must be before they are archived.
Archived email is moved out of your mailbox on our server into an archive file on your computer.
2. Move older email into a folder on the server
A second option is to move older emails out of your inbox and into a separate folder still on the server. This works in any email program. To do this, right click your email account name and select New Folder. Name the folder something like "2024 Email".
While viewing your inbox emails, click the first email you want to move, then hold down the Shift key and click the last email you want to move. This selects the whole range. Generally try to move 1000 or fewer emails at once.
Right click the selected range and select Move, then choose the folder you just created. Repeat until your inbox size is reduced.
More information on archiving: Archive older items automatically (Microsoft Support).