Read more about the opt out link on the following qualtrics page dealing with distribution error messages: https://www.qualtrics.com/support/survey-platform/distributions-module/email-distribution/email-distribution-error-messages/#NoOptOutLink
Qualtrics also recommnets having the people being surveyed add you to their address book to decrease liklihood of this issue: https://www.qualtrics.com/support/survey-platform/distributions-module/email-distribution/avoid-being-marked-as-spam/#AddTheFromAddressToYourAddressBook
Qualtrics is whitelisted on Purdue's spam servers, so nothing should be blocked at that layer.
If you are still having issues with this, please click the 'Purdue IT Request button" and start a ticket, including with the survey and user information.