While we and our email service provider do our best to keep up with industry best practices in email management, it is not a perfect process, especially where spam and phishing prevention are concerned. Every email client has its own methods for identifying spam and phishing attempts, and those methods are constantly changing. Google, in particular, is known to be a fast-moving target.
The message that you're seeing from Google indicates that the message is getting flagged because the "From" name matches a name in your organization but the message is coming from a domain that it doesn't recognize. If Google allows you to whitelist the following domains and/or IP addresses, we recommend doing so in order to ensure emails are delivered: