iPhone automatically places different email address in From field.

When I receive an email to my @me address (which is an alias), then hit reply to respond, my iphone automatically places my @mac address in the From field. Why won't it automatically reply from the same email that the message was received in?

Just to clarify:

I received an email to my email@me.com address. When I reply to the message, my phone automically places differentemail@mac.com into the From field.

So I have to tap the From field to switch it back to the email@me.com.

This works fine on my computer. The email that received the message is the same email that is automatically placed in the From field.

Any suggestions