As Lee stated, there is a good reason you have not had voluntary contact with your mother since you have been 14. Obviously she has done something to make you very wary of her.
Whether or not she is being nice now really isn't the issue. You do not trust her and you do not want her contacting you. Her contact is making you more than uncomfortable.
You can send her an email telling her you do not want her in your life. Or, if you prefer not to be so direct, mark her email as "spam." That way it will be directed to the spam folder and you won't see it in your in-box.
Being contacted out of the blue, especially after so many years is sure to raise one's level of wariness. The questions that come to mind are "what does she want?" "Why did she choose now to make contact?"
You have every right to maintain your boundaries with her. You have to do what is in your best interest, not what is more comfortable for her.
But think back to when she was in your life. Did she play the guilt trip game? Did she do something abusive and then follow it up with "apologies?" Did she try to get you to understand her problems as a way of excusing her abusive behavior?
Do you feel like she is manipulating you again?
If she is contacting you because she has something important to tell you, she will come out and say what her reason is, and not play the "I can't call you, I can't write to you, how do I know it's really you?" game.
You are under no obligation to respond to any message you receive. If you choose to respond, the response does not need to be immediate. Think about how you feel and what you want to say and respond on when you are ready.
Acceptance on someone else's terms is worse than rejection.