A knowledge base article about Why do bmail / email messages get marked or flagged as spam? provided by the UC Berkeley IT Service Hub - Knowledge Portal
UC Berkeley's bMail platform is managed by Google, which partially relies on spam scores for determining if a message is legitimate.
The more often that a recipient marks messages from a sender as spam, the more likely it is that future messages from that sender will be delivered to the spam folder -- even for other recipients.
Note: Google's algorithm for identifying spam messages is proprietary, and therefore it is not possible to provide a detailed list of reasons why a message was flagged as spam.
If you are sending emails and the recipients are reporting to you that your messages are landing in their spam folder, they should follow the steps below.
Locate the legitimate message in your spam folder, open it, and click on the "not spam" button.
This will move the message to your inbox and reduce the spam score for the sender. You may need to repeat this step two or three times for email from the address going to the Spam folder
By flagging the sender as "not spam" you will help their legitimate messages be delivered to others, and by adding them to the "my contacts" folder, you will ensure that their messages are always delivered to you.