Invalid mail address. For password recovery and notifications

[dmitry]

found the fix

Problem on setting an email account for users (always invalid)

You can also fix the error by uploading these two files from previous OwnCloud version 9.1.7. lib/private/Mail/Mailer.php lib/private/Mail/Mail.php

Just as a follow up - is your ownCloud on a hosted server?

Or are you using CPanel?