E-Mail Comment Notifications on Multi-Sites not working for Sub-Sites

I am not sure if my problem is exactly the same as the one mentioned here (https://wpmudev.com/forums/topic/e-mail-notification-for-comments) by EJarolim, but its effects seem very similar.

I now tested, if comments work on pages, as I normally do not allow comments there. But those have the same problem, though I think I got those longer than the comment notifications. (I can’t check that, because a computer crash swallowed some of my not saved mails.)

But I get mail from contact forms and other alerts from the page, just no comment alerts. Those also do not go out to the comment poster, though I would have installed a plugin to get alerts for responses.

On some websites of mine, everything is fine and on two others that just won’t work. As they all are on the same server and in the same service of the hosting provider, I can’t detect any difference there.

I already tried to deactivate all plugins, but that won’t change anything, as far as my tests revealed.

The two sites where the comment alerts are not working, are both Multi-Site installations and the comments would be on sub-pages, where the normal alert and contact mails from form entries work.

For one website, I have a catch all mail address and for this one I got strange error messages. It is as if mails were sent from an address for my website, which does not exist, as WordPress seems to create a mail address after its own logic, like

wordpress@subpage.website.com

The mail address obviously does not exist that way and was never entered by me in that form. I also have no idea where it is mentioned like that, because I could not find it anywhere in the backend, where my normal mail address is everywhere I could find a mail address entry.

But it seems, WordPress has a kind of logic for MultiSite installations since a few updates of WordPress back, because before everything seemed to work fine. But now, it seems to self-create mail addresses for the comment notifications. I searched, but still I could not find out where I could influence those used mail addresses for the sub-pages. My best guess is the information stated in the network-admin site settings, where the information is:

Mail-Server : mail.example.com

Mail-Server login: login@example.com

But here I am not the least sure what I will influence here or how the correct setting should look like.

The website seems to send mails correctly, where the sender mail can be clearly defined or takes the admin mail address. But everything else automated seems to not work properly on the sub-pages, as the comment notifications try to use a non-existent mail address.

After I entered the code for PHP-Mail in the .htaccess

# PHP – MAIL

php_value mail.force_extra_parameters –fwebmaster@example.com

I got the error messages I usually got for comments on the website with the catch-all mail address also for this site, so at least that seems to catch the messages now somehow, though the error with the wrong sender mail still exists on both websites.

I tried to install the suggested SMTP plugin (WP Mail SMTP), but the test mail for the plugin already gives back the usual error that the sender mail wordpress@subpage.website.com causes problems, though I specially entered the access settings for my e-mail, which should be used as sender address, but those settings are ignored.

I am a bit at a loss now, where I could continue my search for a solution now and would really appreciate, if you could help me with this error. Thank you very much in advance !