It's not slander unless the employee made untrue factual statements about the former employer, which statments damage its reputation. But an employee or former employee may not use employer resources or information for his or her own gain; so if the employee used email addresses and contacts he or she only had as part of his or her employment to mail to the company's client list and--implicitly--asking them to keep working with him or her after termination (which is what asking them to contact him/her with questions can be taken to be), the employee did misappropriate the "property" of the employer and potentially could be liable for any losses (e.g. lost clients) they suffered. An employee should not reach to employer clients using the employer's contact list or email addresses provided by the employer after termination of employment.