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Its the main account you're sending emails from. Its just a convenience. You should give us more information, so we can better assist you.
I responded above prior to seeing your reply Ledsteplin. Sorry about that. At least we're on the same page, huh...In my case, I have 6 email accounts. The one they call the default account is the first email account I set up. It was a Gmail account. So now it's just called Gmail. My other gmail accounts go by the full email address. You can change which account you want to be the default account.
Sent from from my iPhone 5