Google's Spam filter will occasionally mis-classifying mail from certain users as Spam. You can make sure it doesn't tag incoming messages incorrectly in a few ways. You can help Google's Spam algorithm by telling it directly that a message isn't spam. When viewing messages in the Spam label, click the Not Spam button to move the message into your Inbox and tell Google that it shouldn't be marked as spam.
If a certain user's messages are still getting sent to the Spam label, add them to your Contacts. Mail from users in your Google Contacts is not marked as Spam. Email addresses you reply to are automatically added to your Contacts. You can add a user to your Contacts when viewing their message by clicking the down arrow next to the reply arror, and select "Add <name> to Contacts list."
Learn more about Google Contacts.
Filters can also rescue a message from the Spam label. Create a filter for the affected message under More > Filter Messages Like These >Create Filter with This Search, and select the "Never Send it to Spam" label.
Learn more about filtering messages.
Andre
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Andre Schmidt
Project Manager, Emerging Technologies
Office of Technology Services
Illinois Institute of Technology