Sometimes Gmail might incorrectly mark messages coming from senders outside of your domain as spam. As a G Suite administrator, there are a few things you can do to quickly fix this problem and prevent it from happening in the future. 

Take the following actions when Gmail incorrectly marks some or all of your external incoming email as spam.

Follow these recommendations and encourage all your users to do the same:

1.Mark incorrectly classified messages as Not spam

Gmail uses machine learning to better understand your preferences and what constitutes spam. When you manually mark a message not spam, you teach Gmail how to handle messages addressed to you. 

2. Add trusted senders to your personal contacts list

Users in your domain can manage their personal contacts list in Gmail. When a user adds an external address to their contacts list, it lets Gmail know to not mark their email as spam.

Add trusted senders to the spam bypass filter

Whitelisting is adding a domain to a list of approved senders bypassing Gmail spam filters. Adding a domain or specific email addresses to your G Suite spam filter reduces the risk of future email from the sender not arriving to your users’ inboxes.