Hi Simmi,
Could you share screenshots about the mailbox and it's status?
The error indicates that the email will attempt to be synced again (up to 5 times if failures continue) before giving up. Its most likely there could be an interruption to the sync that requires another attempt.
You can check the owner of the mailbox if have a license or security role.
You can changed the owner of the queue and the mailbox with user having valid license and roles.
