When tensions are high during a divorce or custody battle, it might be best to communicate with your spouse by email. This avoids angry face-to-face encounters or arguments on the phone, where you have more opportunities to push each other's buttons. Emails permit you to consider the message and take some time to provide a thoughtful response, if you take that opportunity.
Remember, whatever you say to your spouse might be read by a judge one day, who will be making decisions about whether you share in custody of your child, how much child support you may pay, what portion of the property you will receive and whether you will pay or receive spousal support. Before you hit the "send" button, consider whether this communication really sends a positive message to your spouse and represents the image that you want to present to outsiders.
The best thing to do, barring an emergency, is write the message one day, sleep on it and then review it the next day. If it still conveys an effective message after you have considered the situation more carefully, then send it. If you are involved in a formal divorce, it would serve you to create a folder for your communications with your spouse so there is a record.
If you are really angry with your spouse and see your communication becoming worse, consider asking your counselor or lawyer for help in drafting responses. When you receive an inflammatory email from your spouse it's easy to want to respond in kind. You may be set up. Your spouse may be baiting you into lashing out. If you take the bait, you look bad. You also continue in a destructive game that doesn't benefit your family or you. A professional will be able to help you respond in a considered, productive way, rather than perpetuating the fighting dynamic that brought you to the divorce. You will receive credit from the judge for being smart enough to recognize the damage the old dynamic was doing and taking steps to correct it.
Two wrongs don't make a right, particularly when you are creating a written record.