If your emails from Infusion are being marked as Spam please check your email defaults.
If you are sending using an unauthenticated relay, (not having the tick for Require Username and Password), it means that anyone can use that email server to send emails which causes the servers to get blacklisted quickly.
Once the Require Username and Password is selected, these can be populated. This way all of the SPF / DKIM / DMARC records are in place and it is less likely the IP of the mail server is blacklisted.
But, if your emails are still getting marked as spam, run a test email using mail tester. It will give you an email address to send to and rate the email out of 10 for spam. It is always best to do a test invoice and email it from the Queue to the address so it is a real test.