You can do that on email no problem, and its common as heck for major email providers to block on little more than a mail server being in the same IP block as a

You can do that on email no problem, and its common as heck for major email providers to block on little more than a mail server being in the same IP block as a bunch of spammers. Its not the same you say? Yet no email admin is prevented from doing things as you describe.