Problem:

Because of the SPF problem, emails are sent with the noreply Address.
If a user replys to that mail, it will not work.

The Solution would be to set the reply-to header, which is also implemented in the email_to_user function in lib/ moodlelib.php. But those optional parameter is nearly
never set in function calls.

Proposed Solution:

Set the replyto-address as default to $from->email at the begin of the function email_to_user:

    if (empty($replyto))