This can happen for many reasons, such as sender spoofing, mass emails, someone flagging messages as spam, an IP address used to spam, content that triggers AI that the message is spam, domain has become blacklisted, etc.
G Suite Spam Settings
There are a number of spam settings that can be configured to help prevent incoming and outgoing spam.
- See Spam Settings - G Suite Admin Help
- See G Suite marks valid email messages as spam - G Suite Admin Help > Action external senders can take.
Message sent to Gmail users are marked Spam
The number one fix to prevent outbound messages being marked as spam is to add DNS records (SPF, DKIM, and DMARC) to authenticate outbound mail messages.
For more information on DNS record changes:
- For SPF see Help prevent email spoofing with SPF records - G Suite Admin Help
- For DKIM see 1. Generate a DKIM key for your domain - G Suite Admin Help
Outbound Message Gateway
If the domain is using an outbound message gateway, DKIM may be invalidated.
- See section How do I set up DKIM for a server that modifies the content of outgoing emails?
- See also Set up an outbound mail gateway - G Suite Admin Help
- For DMARC see About DMARC - G Suite Admin Help
Note that DNS changes take some time. Expect that some messages may still be marked as spam for up to 72 hours. See https://support.google.com/a/answer/7514107?hl=en
Senders should follow bulk message guidelines.