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

[Werner Weissenfels]

Hi,

can anyone tell me where I can set which "from" email address typo3 uses 
when sending emails?
This applies to the test with the install tool and also to the 
fe_user_registration extension and possibly other situations where typo3 
might send emails.
In none of these situations sending email works for me.
When checking the mail log I found that invalid "from" addresses cause 
the mails to bounce when Postfix is trying to send them.
I wasn't able to locate where I change the "from" address.

Thanks for any help,
Werner