What is the purpose of the "Reply-to" line

Say I want to send a message from account "Test1@test.com" and they reply to account "Test2@test.com" I would put Test2 as my reply-to by test1 as my From account so the headers come out as this.

From: Test1 <test1@test.com>
Subject: Test
Date: February 22, 2011 3:29:36 PM PST
To: Whoever <stevejobs@microsoft.com>
Reply-To: test2@test.com

So when they click the "Reply" button it -should- put their "To" field as the test2@test.com

This is how it is supposed to work but does not necessarily mean all email clients and providers will treat it the same and they may still end up replying to your main email address.

Typing a valid email address in the "Reply-To" line should be the one that auto-populates the recipient's "To" field when (s)he replies to your original email. Have you tried sending an email to a second email account of yours, and upon receipt, click the "Reply" button to see what address auto-populates the "To" field of the reply? If not what you specified, it would be helpful for you to look at the raw message headers of the original test message received at that other account and see if the "Reply-To" header exists. (In Mail.app, from the menu bar, it is View > Message > Raw Source).

In the recipient's list of email messages in his/her inbox, the message will show up there as being from the actual sending address; the reply-to address should only come into play when the recipient hits the reply button.

Just to add a "post script" to the above, not only will your actual sending address be what is shown in the recipient's message index list of his inbox, but also when the message is viewed in a preview pane or opened and viewed in its own dedicated window. Furthermore, even though the reply-to address should only come into play and populate the recipient's reply message's "To" field when the recipient hits the reply button, there is no assurance or mechanism to prevent the recipient from manually changing the "To" field" back to your original sending address.

Again, a post of the raw headers from the message as received by your recipient, and perhaps even from the message in your sent folder (Mail not set up on this particular computer so can't see if "Reply-To" is in sent headers, seems like it should be, but this computer only has Outlook Mail configured which apparently lacks this functionality, so can't check this right now), in order to determine whether the "Reply-To" header is being removed somewhere along the way, would be helpful to further troubleshoot your issue.