Yahoo user name interferes with forwarded emails.

FFor some reason my IPad required me to use my Yahoo User Name followed by "@yahoo.com when I set up my mail account. My user name is different than my e-mail address. It only causes a problem when I forward an e-mail. The recipient receives the e-mail from "username@yahoo.com. I can not seem to find out if this can be corrected. Any help would be appreciated.