Cook sent the note exclusively to Cramer on Monday before the opening bell after being questioned by Cramer about Apple's performance in China.
"I get updates on our performance in China every day, including this morning, and I can tell you that we have continued to experience strong growth for our business in China through July and August. Growth in iPhone activations has actually accelerated over the past few weeks, and we have had the best performance of the year for the App Store in China during the last 2 weeks," Cook said in the email.
Cramer said that despite a weakening in China's economy, "this consumer company, which has a very big percentage of business in China, is experiencing acceleration there. So we have to compute that in," he said.
Read More Analyst: We like Apple on this weakness
"To me the truth is Apple is doing better," he said. "I don't fear systemic risk in the U.S."
His advice: don't panic and do not get picked off. "Everything must be limit orders. Do not issue a market order, you will be fooled at the end of the day."