E-mail distribution problem

Hi there,

I'd like to pick your brain on how to tackle a problem that me and my company is facing.

We are sending an e-mail distribution twice a month to our clients.Recently one of our clients raised a question regarding our method.

All of our mails are sent in the clients design and the sender of the mail is an employee of our client. The catch is that the mails are sent from our mailserver and therefore the specific clients IT security department is worried that we're accustomising their employees to trust e-mails considered to be phising attempts.

My question is, how can we tackle this problem? I welcome all possible solutions.

Thanks for your time,

John