A disclaimer is not really what you need, since all it would say is something along the lines of:
Party xyz (you) is not responsible for the content of e-mail's that originate from customers of this site, and any opinions expressed theirin are soley the opinion of the sender, not the site. e.t.c. blah blah.
What you really need is a terms and conditions of service page which outlines what is acceptable and unacceptable behaviour of clients of this site. State e-mail content guidelines in there, e.g.: it is unacceptable for a user to use this service to send abusive or offensive e-mail. You should have a list of all of the things that will get a users account terminated.
You also need to state what you mean by free, as false advertising in this country (UK) is illegal (although from some of the ISP shenanigans going on at them moment concerning 24/7, access you wouldn't think so), and if they can be arsed to the ASA and Trading Standards will kick your butt.