How can I find out what led to a message being identified as spam
If you are sending from off-campus, your host may be on a block list. The spam scanning service utilizes third-party block lists when determining the probability that a message is spam. Many Internet Service Provider IP addresses are present on such lists.
As a Purdue-affiliated user, you do have the option of configuring your e-mail client, so that e-mail sent to @purdue.edu e-mail addresses is not quarantined. This is done through the use of SMTP Authentication.
Information on how to configure many commonly used e-mail clients can be found on the webpage:
Mailhub Client Setup