Trouble Sending to Gmail / Google Workspace domains

I have been having the same issue. Certain clients who use Microsoft cannot send us emails due to Google Workspace. It is saying that our system is blocking them despite us changing all of the spam filters to allow for their emails. The last thing we are trying is to whitelist the IP addresses of Barracuda and Microsoft.

Here is the message our client's see:

Status code: 550 5.7.350

When Office 365 tried to send the message to the recipient (outside Office 365), the recipient's email server (or email filtering service) suspected the sender's message is spam.