On June 1, Art Dealer mailed an offer to Collector. The offer, which was received on June 3, offered to sell a painting to Collector. On the same day he received the offer, Collector mailed an acceptance of the offer to Art Dealer. On June 5, Collector called Art Dealer and told him that he had mailed an acceptance, but was calling to revoke it. On June 6, Art Dealer received the acceptance. Under the Mailbox Rule, an acceptance is effective when mailed. However, an exception exists if a revocation of the acceptance is received by the offerer before the acceptance is received. On the facts above, was Collector’s acceptance effectively revoked?