The most common reason for this is bloat. If you keep every email you ever get, and never organize them into other mailboxes or archive very old messages, your email could be massive after several years.
It is highly recommended you keep no more than 100 emails in your inbox. In should really be for new and urgent items. If you are done with an email, either delete it or file it in another mailbox for safe keeping.
Any given mailbox should not have more than a couple thousand messages before you think about archiving a large portion of it or just deleting things from several years ago.
If you keep up with your email filing, you will avoid issues and be able to find things faster when you need them. If you need help doing things like making mailboxes/folders, or making email filters to automatically sort incoming mail, contact your support person.