Sometimes an email from a Pima College student, whose email address resembles user@mail.pima.edu, may include a warning banner like the one shown here:
This warning banner indicates that Google’s spam-detection scanning found something in the email that would ordinarily send it to your Spam folder. However, the email was not marked as spam because the College has rules that keep emails sent by Pima students from being routed to your Spam folder.
These rules are necessary because pima.edu and mail.pima.edu are two separate domains, which means student email qualifies as “external” and is therefore subjected to more rigorous spam filtering. But the rules that keep those few emails which scanning identifies as spam out of your Spam folder also cause the system to display the warning banner.
Google’s spam detection algorithms can tag an email as spam for a variety of reasons, but Google does not disclose what those reasons are, because publishing those details would make the algorithms easier to circumvent.
Just because Google’s algorithm saw something suspicious does not mean the email is spam. However, it’s always wise to be cautious about links in email.