when dokuwiki sends an email (e.g. at subscribers of page changes), it did not set the envelope sender address correct, because it does not specify it. Its up to the hostsystem to set an apropriate address, which is, in most cases, not what the admin wanted, especially if he specified an sender address to be used in dokuwiki's configuration.
The function in question is "notify()" in "inc/common.php". There at the end of the function one can see:
This should be changed to:
which will inform the underlying transport agent (mostly postfix or sendmail) to use the "$from" (=configured sender address) as envelope address. If this is not given it will use something like "firstname.lastname@example.org