Smerconish, a lawyer, said on the air that he believes the email constitutes evidence that "jeopardises the criminal case" against Cosby.
The contents of Cosby's 2005 deposition in a civil suit involving Cosby accuser Andrea Constand - in which the comedian admitted to drugging women with whom he hoped to have sex - are key to the current criminal case.
Last week a preliminary hearing was put on hold after Cosby's lawyers filed to dismiss the charges.
Celebrity lawyer Gloria Allred, who represents 29 Cosby accusers, including Judy Huth, a woman who filed a separate civil lawsuit against the comedian, said in an interview that she's "not unduly concerned about a complete dismissal of the [criminal] case."
"Obviously it's something a judge has to decide," she said. "But I think there are very significant arguments against the defense's motion. For example, why would Mr. Cosby's lawyers have relied on an alleged oral agreement of non-prosecution and not have it reduced to writing, when it was so important to Mr. Cosby?"
More from Spy:
• Pregnant TVNZ presenter 'an eyesore'
• Manu Bennett shows off mermaid moves
Allred also questioned whether the email might have been politically motivated.
At the time it was sent, Castor was running for district attorney against Ferman's deputy, Kevin Steele. Steele was ultimately elected in November and filed the criminal charges against Cosby in December.
Cosby's accuser Constand is a former Temple University basketball coach who visited Cosby in 2004 at his suburban Philadelphia home, where the alleged sexual assault took place.
Cosby settled the civil case brought by Constand's lawyers in 2006.
- Washington Post