Getting a SMTP server response: 550 error when sending a form

Hi guys

I’m having some issues using an email address as the receiving party… Does this error mean anything to you?
“Warning: mail() [function.mail]: SMTP server response: 550 Requested action not taken: mailbox unavailable or not local in D:\Internet\root\www\(redacted)\httpdocs\(redacted)\wp-content\plugins\formidable\classes\models\FrmNotification.php on line 372”

My own address works fine, but with any of the companies addresses I get this message.
So it will probably be the case that there is something wrong in the settings of their email but via google I can’t find a specific answer (I get loads of different results) so maybe you’ve handled with this before and know where the issue lies in connection to your plugin…?
Thanks in advance!