My emails are considered as phishing

Hi,

I have a customer that I send a lot of emails to. I use Mail.app 5 and I have four domains with different email accounts. All my emails I send to him is considered as phishing by his email client (Windows mail). Even if he clicks trust this email address, the phishing warning continues. He receives hundreds of emails a week, but only mine is considered as phishing.

I thought it was maybe my web hosting SMTP that was the problem. But now I changed to Google apps and their SMTP, and it's no changes.

As I send so many emails in my job I'm afraid more people then him is getting the same warning, just that I don't get to know about it.