[TYPO3] Where to set from email address for mails sent by typo3?

[Werner Weissenfels]

Gideon So wrote:
>     Add a hidden field with field name: email.

Thanks. And where do I add such a field?
My question is not about using forms with email (I know that there I can 
add hidden fields).
Where do I control the from-address for the test mails I can send from 
the install tool?
And where for the emails that can be sent by the fe_user_registration 
extension?

BTW, this is a new (typo3 4.0) installation and it hasn't worked ever.


Thank you,
Werner