I have multiple domains that I manage where we'd like to send e-mail from. For example: user1 has a reply-to address of user1@abcco.com. user1 would like to send as user1@organization.org. I don't want to have to create a new account for this user (such as user1orgemailaccount) and then give them full permissions and send as rights. Any ideas?

You have to unless you buy some software that alters the address on the fly. when Exchange sends mail it always uses the primary SMTP address as the sender of mail.

See http://ivasoft.com/choosefrom.shtml