I’ve been chasing my tail for DAYS on this issue. Everything from “Message.php” and “Mail.php” config file changes to issues with PHP 7.0
"Can't send mail to inform the user about an internal share: Address in mailbox given [@] does not comply with RFC 2822, 3.6.2."}
None of that helped. Come to find out the user account I was logged into Owncloud with was an LDAP user and had no email address in active directory. I added a null email to my AD user, logged out, logged back in to Owncloud and voila! I can share folders and files and “notify user” without any problems.
Such an easy (but dumb) solution.