Not exactly. Some methods won't be nearly as effective (such as filtering on the message body) but others (such as SPF, DKIM, and RBLs) will still work just as well as they do today.

Now that I think about, just the encryption itself will increase the computational cost of sending out spam e-mails. While today a spammer can blast out an e-mail to 100 recipients very quickly, it'll take a fair bit longer to do once the spammer has to query and retrieve 100 public keys (one for each of the recipients) and then encrypt the e-mail 100 times over.

An intermediate solution?: All encrypted email senders have to be while-listed by the receiver. All clear email will be allowed by default, and usually be the first way of reaching somebody, either for legitimate or illegitimate (spam) purposes.