Customer Sign Up Issue SMTP - Error

[CaptainFacePalm]

Hello Everybody,

I am running v139h and PHP Version 5.2.6. I have the following mods installed are:
- Capatcha Code
- BOGOF
- Dangling Carrot
- Include & Exclude Tax In Prices
- Mailchimp
- Customer Tax Exempt
- External Polling Sidebox

So I have got that out of the way the issue I am having is when a new customer signs up to our website when they finish creating there account it comes up with Email Error: SMTP Error: The following recipients failed:

What would be causing this to happen I was thinking it might be the Mailchimp add on. But not sure any help would be great also if you can solve this for me I will buy some love sticky yummy doughnuts and coffee. Also I have included and image of what a new customer sees.



Cheers,
Thom