Do you mean emails via the contact form?
If so you could try a more complicated captcha (e.g. an image-based one) to see if that makes a difference.
Let me elaborate: what I mean by "more complicated", is something that might not be so easy to crack.
Your current captcha asks the user to add two numbers, e.g. 6 + 11.
eval("6+11") to get the correct answer.
Also, another good method I heard of to avoid spam on contact forms, is to create a hidden field (i.e. a normal text input field that you move off of the screen with
left:-9999em or something.
This will be invisible to normal users, but visible to spam bots, who generally won't be able to resist filling it out.
You can then filter mails on the basis of those which contain an entry in this field.
I'm not sure of the correct name / implementation of this, but I'm sure that you'll be able to find something on Google.