Just keep in mind that read receipts are not part of the POP3 protocol, so any recipient (even those using Outlook) can deny or in some cases, will not respond to a read receipt request. To be honest, they're just about useless unless you're working on an Exchange-based system. Even then, if your administrator doesn't lock it down, people will be notified that there's a read receipt and can choose to acknowledge or deny it.
Also, sometimes spammers use read receipts to determine whether or not there's a living person on the other end of an email address. If you acknowledge spam, this will get you placed on a "gold" list of more desirable email addresses and will like incite more junk.