Emails will show up in your unmatched folder for a number of reasons. For instance, if the emails you are receiving are from addresses that aren’t attached to an opportunity in your project. 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. Hence, their response will go to the unmatched folder.
To learn how to match these emails to opportunities read “How to match an unmatched email”