Since you have only about 10% of these stuck, i’d say this is normal. This means that the recepients email address wasn’t found. It could have been either a typo or a problem at the receiving end.
You can test if these are good emails or not by emailing from a third-party email service like yahoo mail or gmail to the failed address.
Sounds like there might be a Cisco firewall in play here, try toggling the SMTPfixup portion of your cisco firewall, if you have one… sometimes this gets in the way.