Partial states are typically temporary and may indicate issues with the mail server. Accounts in this state, however, are still running.
One or more folders in the account may not fully sync currently or not sync mail at all. Typically, accounts recover and return to a full-running state.
Some reasons an account may be in a partial state:
It's a new account and hasn't finished its initial sync.
The account was recently reauthenticated.
The user deletes, changes, adds, or restricts folders.
Slow network connections.
Low bandwidth on the email server.
A backlog within the email integration service.
A large number of folders in the user's mailbox.
Syncing historical data
The user has external integrations that may be slowing down their email server.
The user’s email server has connection or network issues.
Microsoft accounts with numerous folders.