Emails will show in your unmatched folder for a number of reasons. If the emails you are receiving are from addresses that are not attached to an opportunity in your project, they will appear in the unmatched folder. In some cases, this can include emails unrelated to your outreach such as work and personal emails.
In other cases, these emails may pertain to your outreach. Occasionally, if you reach out to an opportunity but a colleague of theirs responds instead, that contact might not yet be associated with the opportunity and their response will go to the unmatched folder where a new contact can be added during matching if necessary.
To learn how to match unmatched emails to opportunities, please read our article on how to match an unmatched email.