Does that really help, when the spammer can make their own email account on the target's email service (eg. Gmail), send the spam email to this email account and see if it gets through the filter?
I don't know. Although I would be mildly surprised if there was a single universal spam filter applied to all Gmail accounts. I would also think there are more inputs to the system than just the sender and the body of the email. Gmail being so big allows other variables like the rate that address is sending to all of Gmail as well as how users engage with those emails. Therefore getting a single test email through the filter probably isn't proof that you have defeated the overall filter.
