Find all/ Replace email addresses

Our company was acquired by a larger company and all email addresses were changed from one ISP address to exchange server addresses. Is there an easy way to search for and replace “all” email addresses *@company.com with *@newcompany.com – where, * is the username? The exchange server has a global list of 90,000 addresses. Sufficient time was given to users to correct addresses but since the old addresses still “worked” in the interim period, many did not correct or did not know how to correct the addresses.