I received a lot of complaints from my users, they say that they can't find the activation email. I didn't do any blast emails or send spam emails. But I do send over 1000 emails daily, that's because of the signup. That means, in my registration process, I will send an activation email to the user to activate their account, and I think this is quite a standard method.
I tested my company email and gmail accounts, it works fine, only hotmail and yahoo emails, they just could not receive any emails from our website, not even in the junk/spam box.
Can anyone help me on this?
My server: Windows Server 2003, IIS6
Programming language & database: php and mysql