Mailbox unavailable.: When trying to send emails to google apps email account,Error

[ShyjuK]

I have an asp.net site hosted with parallel plesk and i am trying to send an email from an asp.net page when a user reisters with my site.I have enabled google apps for this domain.So i can access email there.When i am trying to send an email to one email account created in my google apps (ex : admin@flarack.com) The email sending code is throwing an error likke
" Mailbox unavailable. The server response was: Requested action not taken: mailbox unavailable or not local"

But if i change the to address to some gmail id, it works well. Can any one tel me how to solve this issue ?

Thanks in advance