well, if your isp is asking for ip justification, that means they give out ips only for ssl purposes. if you tell them youre going to be mailing it is more likely that you do not get the ips that you ask for.
so tell them its for ssl, and host subdomains on those dedicated ips and install your self signed open ssl on those ips, and use them for mailing. you install the openssl so that if they check at a later time, they would find an ssl in place. Thats the only way to do it. otherwise go offshore where companies give you ips without questions.