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I am afraid there is little I can do about it. It is a combinations of the email address I use for the site, the settings of the mail server coming with the domain and the mail server receiving messages. I still receive all notifications of PM's so I know mail sending is not down (and we still have new registrations and these would be there either if email were not sent).
Main culprit of people not receiving email notifications is that their mail server is set not to accept mail from a server that does not correspond with the domain given in the email sender's address.