There can be various reasons because of which the emails get stuck in the outbox. As soon as you login to the Dashboard, you will see a notification that says emails are stuck in the outbox.
Once you open the Outbox, you will see the exact error message. Most of the times it’s a bad import or an extra dot after the email address.
So if the CSV file is not proper or if you match wrong fields with wrong merge codes this can happen.
For example, matching URL with the email address. If you go to outbox, you will see a generic error message and after editing the problematic email, you will see that receiving address is a url.
