/admin/Settings/x:MtaStageAuth
just killed my setup of Nextcloud e-Mail, as the eMail-setup in /index.php/settings/admin was set to a “From” address that is a sub-address for a mailing-list (i.e. mailing list “system”, subaddress system+nextcloud), which goes into the sys-admin’s mailboxes when responded to. But the login is a default user account which mailbox will never be bothered about.
Generally speaking, if I cannot send with the “virtual” sender I’ve set up in mailing list or aliases, this is a hidden neck-breaker. I strongly suggest to consider to switch the default setting must_match_sender = false. And/or allow override in /admin/Management/x:Account/User/