Inability to use cc or subject line when composing email?

If I start a new email and then cut and paste an address from another app (say from Excel), the cc and subject lines are suddenly inaccessible. I can get to the body of the email but I cannot type or paste anything in the cc or subject lines. This is an important issue - I always prefer copy and paste to typing to avoid typing errors. But I also need to be able to fill in the subject and cc lines. Please tell me how I can do all this with a new email. Thank you.