I used my former employer's customer e-mail list to try to drum up my own business. How much trouble could I be in?

Assuming you have no non-compete provision in your employment contract with your former employer, it depends on whether the customer list is considered a trade secret or not.  Some customer lists are considered trade secrets, and some not.  It depends on factors such as whether the information was publicly available, the time, effort, and money expended by your former employer to put the list together, how extensive the list is (i.e. is it just name and number, or also contact person, preferences, ordering history, etc.), what efforts your former made to try to keep it secret, etc.  You are not allowed to misappropriate someone else's trade secret for you own use.  You also can't work for your own business on your employer's time; from what you've written you didn't do that, you downloaded the list for your employer's purposes and only decided to use it later, after you left.  But that may not be the way it looks to a neutral third party. I do think it is unusual for the police to be involved; the type of circumstances you describe are much more often the subject of a civil suit, in which you could be enjoined from using the list, and also have to disgorge any profits you made from the list.  Again, the key question is whether the list qualifies as a trade secret, which can't properly  be evaluated without a lot more facts. With all due respect, I think you were naive to think that your former employer would not challenge you because of the small size of your business.  If the customer list is a trade secret, it will only remain so if your former employer takes reasonably steps to protect it from use and disclosure by someone else.  If you allow someone to use your trade secret for free, you may waive trade secret protection.  Also, regardless of your small size, your former employer wants to send a message so that nobody else tries to do what you did.