Any of your forms will be subject to spammers. Comments, subscribers, contacts, etc.
Are you able to get bounced email message reports for your subscribers? I've found that's a great way to remove any emails that have snuck through my traditional anti-spam measures. If it's returned... it's removed.
Depending on the system you are using for subscribers, make sure you enable or add spam prevention if you can. Captcha is a pain but a necessary evil sometimes.