Just get rid of the account wrapper you put around the settings.
nail.rc
Instead of:
# other mail config ... account gmail { set smtp-use-starttls set smtp-auth=login set smtp=smtp://smtp.gmail.com:587 set smtp-auth-user=username@gmail.com set smtp-auth-password="blahblah" }
Use:
# other mail config ... set smtp-use-starttls set smtp-auth=login set smtp=smtp://smtp.gmail.com:587 set smtp-auth-user=username@gmail.com set smtp-auth-password="blahblah"
It will use those as the default settings.