corect way to use mail() function

Hi, The company I work for, hosts online events registration applications. After a registered registered himself for an event he will get a confirmation email saying he registered successfully. Currently, in the header part of the mail(), in "From" it says e.g. orders@computility.com - because the email comes from us not from our client (e.g. ABC Assoc.). Reply-to goes to us too. Now one of our clients (e.g. ABC Assoc.) asks us to put in the "from" field their email, to looks like the email comes from them. something like: From: ABC Assoc. <events@abcaccos.org>; I refused to do that concerned we are going to be blacklisted for sending "spam". Because header shows one place and From field says other email address - spam way of sending emails. Am I right or it really doesn't matter "who" sent the email? LL