Freemark Communications, a Cambridge, Mass., provider of free, ad-supported e-mail, is searching for a new CEO after founder Robert Young stepped down. Mr. Young will continue as chairman. Ameritech recently invested in the company and is believed to be exerting more control over its management. Freemark has about 20,000 subscribers who see ads when they send or receive e-mail messages. Competitor Juno Online Services currently has about 570,000 subscribers.