If the sender is getting bounce-back messages (undeliverable error), the issue might not be on your end. It could be that your company's IT department (if using a Workspace account) has blocked the domain, or the sender's email provider has poor reputation and is being blocked by Gmail's server-level security.
Note: If you cannot find an email from them, you must log into Gmail via a web browser on your computer or phone to use the Settings method listed above. gmail unblock sender