Can't send e-mail - problem with SMTP/MimeMail?

So I've placed the following down, with the appropriate e-mails removed for this post:

mimemail(
  'sender@email.com',
  'recipient@email.com',
  'Subject',
  "Test!",
  $plaintext = NULL,
  $headers = array(),
  $text = NULL,
  $attachments = array(),
  $mailkey = ''
);

and I recieve the following error:

warning: mail() [function.mail]: SMTP server response: 553 We do not relay non-local mail, sorry. in ...\modules\mimemail\mimemail.module on line 331.

I know this isn't a problem with the SMTP mail server settings alone, for I can send a test e-mail successfully. It's only when I use the MimeMail function to create an e-mail. I've tried applying the patch in http://drupal.org/node/132903, but to no avail (actually I'm not even able to send a test e-mail using SMTP after applying it).

Is this a problem on my end, or a conflict between the two modules? Any ideas on how to tackle it?

Thanks!