Normal mail ends up in spam folder

Hi,

how come that are also normal e-mails end up in the spam folder? There is nothing special in the mail, no blacklisted mail or domain, nothing strange or uncommon in the mail. However, it's marked as spam. How can I prevent this? I can send you the original header (better if in private) to sort the problem out.

I can not whitelist the sender, because it's only one of my many customers and i cannot whitelist all of them.

thanks,

marc