DDT on "[Plugin: Contact Form 7] Some email addresses work, some do not?"

Hello,

I am having a strange issue with CF7 2.4.5. The formdata isn't being sent to my client email addresses (their business nor private email address doesn't work, except for their gmail address).

But when i use my own email addresses i do get the email message with formdata.

What could be the problem and how can i fix this?

http://wordpress.org/extend/plugins/contact-form-7/