Can a employer sue me for contacting clients

Absolutely if you breached any common law duties of loyalty or confidentiality. You have a common law duty of loyalty to your employer. Even after you leave you can not disclose confidential information which you were privy to while you worked for a former employer. Even if an employer does not take reasonable steps to keep client lists confidential, once you are on notice not to contact clients your best option is to stop. Since you say the employer "would get an attorney" it sounds like you no longer work for this employer. That's all the more reason to stop. In some cases you might owe not just your own attorney's fees but in many cases employees find themselves having to reimburse their former employer for its attorney's fees as well. You should consult an employment lawyer immediately for details.