How do I stop my ex from snooping into my e-mail or deleting my e-mail account as he has threatened to do?

My ex-husband has been snooping into my e-mail, and was able to do so by logging on and saying that he forgot the pword answering my secret question and after doing so, he even sent out e-mails from my account. This never has been his e-mail account, he has his own Yahoo account. I changed my secret question/answer but am afraid that he will be able to find out the answer to that one as well. Now he has threatened to get back on there and close my e-mail account completely, knowing that by doing that, it would devastate me. I have a Tupperware business and I depend on my e-mail for that, I am also military reservist, and this e-mail is used by them to send me important information regarding my weekend drills and things. I also have lots of saved e-mails from friends and family that were sent back and forth during the months that I spent in Kuwait after being called up for active duty. How can I stop him, and in addition to stopping him, can I report him for this abuse?