In practice, it violates the DSS. In theory, it could possibly not, but that's pedantry rather than reality.
DSS Requirements 3.4 ([Encrypt] PAN data in storage) and 4.1 (Encrypt PAN data across public networks) are generally violated by SMTP mail. Each mail hop is a store-and-forward gateway that writes mail to disk even if only temporarily; unless it's encrypted, that's a 3.4 violation. Each mail connection that's encrypted with TLS is okay by 4.1... but a merchant can't guarantee that your system or the systems between you and them will use TLS, so that would never pass an audit.
While it's theoretically possible to have a fully encrypted path (each mail server with encrypted disk and all network connections protected with TLS), it's unlikely and unenforceable for Internet-based email. So, no, sending card data via email violates the PCI DSS, and you should not have a merchant ask you to do it, and you should not do it if they ask you to.
(It still happens. And PCI isn't structured to make it easy for cardholders to complain about the practices of merchants they deal with. Refusing and moving on to another merchant is probably your best bet.)