From what I understand of the applicability of PCI-DSS 3(PDF), if you actively reject (i.e. don't accept credit card data via e-mail), your e-mail system isn't considered part of the
Cardholder Data Environment. It sounds like you have to be very strict about this - 100% compliance, or your e-mail system is now part of your CDE and has to maintain PCI-DSS compliance.
I'd recommend that you institute a policy to discard e-mails that have any credit card data in them, and create an new e-mail to the customer like you're doing. The difference here is that you absolutely can't accept credit card data through e-mail and use that data for verification or processing of payments. Have policies in place for your customer service agents that they can not solicit card data via e-mail or ask for it to be returned in an e-mail, including as an attachment. If possible, I'd put a filter on your incoming e-mail that looks for CC number patterns (starts with 4, 5, or 6; 16 digits, may be split in 3 or 4 groups...), and removes the CC info.
Keep in mind that I'm not a PCI professional, but have been subject to it for several years. It's always better to play it safe considering the liability, although if you don't allow use of e-mailed credit card information your liability would go down significantly.