ISP says Im sending out spam. Help!!

Log in to your ISP mail account on the website and check for unauthorized activity. If you find any, change the password. It should be a random string of at least 10 upper- and lower-case letters and numbers. Any password that you can remember is too easy to guess. It should be stored in your keychain and filled in automatically when needed.

If there are security questions on the account, the answers should also be random strings—not the name of your dog or anything like that—and make sure you don't lose them. Never use the same password or security answers for more than one account.

Otherwise, are there any Windows or Android devices on your network? How is your Wi-Fi network secured? If you don't know, hold down the option key and pull down the Wi-Fi icon in the menu bar. You should see this:

Security: WPA2 Personal (or Enterprise)

If you see anything else on the Security line, the network is wide open and anyone could be using it.

Someone might have hacked into your account or stole your email address from someone else's contacts list and is using your email address to send out spam. That happens quite often as these people do like using legitimate email addresses as return addresses. Point that out to your ISP.

Please see my revised comment.