Did Soldau create a contract by mailing the release?

Organon fired John Soldau. Then the company sent to him a letter offering to pay him double the normal severance pay, provided Soldau would sign a full release, that is, a document giving up any and all claims he might have against Organon. The release was included with the letter. Soldau signed it, dated it, and took it to the nearest post office, where he deposited it in the mailbox. When he returned home, Soldau discovered in the mail a check from Organon for the double severance pay. He hustled back to the post office, where he persuaded a postal clerk to open the mailbox and retrieve the release he had posted. He then cashed Organon’s check and finally filed a suit against the company, alleging that his firing was age discrimination.
The federal district court gave summary judgment for Organon, ruling that Soldau’s acceptance of the proposed release was effective when he mailed it, creating a contract. He appealed.