So a person can make infinite amount of accounts with an invalid mail using the same IP with ? 
As long as it hasn't been blacklisted? Or is there any limitations? Like 10 account per IP or 1 account per day per IP?
I do not know because I have not tried it before, if the spammer is using different valid emails, it is very possible that it will work unless the person IP has been blacklisted. The spammer may get another BPN to continue his shitty work. So people know a means to also create several emails within a short period of time. As long as it hasn't been blacklisted? Or is there any limitations? Like 10 account per IP or 1 account per day per IP?