How to change the Email sender?

I checked and you are using 2.7, so this will work for you
https://www.ads-software.com/support/topic/212442?replies=2

Of course, you have to remember the change you made, every time you upgrade.

But, which part of the php code should i change?

As mentioned in that thread, look for /wp-includes/pluggable.php lines 354 and 976

That means:

Line 354 $from_email = 'wordpress@' . $sitename;. This line of code displays as [email protected]. Change wordpress into admin, or whatever username you have as one of your domain accounts.

Line 976 $wp_email = 'wordpress@' . preg_replace('#^www\.#', '', strtolower($_SERVER['SERVER_NAME']));. Again, change wordpress into one of your accounts.

That means, the first part of your account only – username – without the @domain.com part.

Thanks a lot!!! I owe you one!