Hi Will,
I just tried to play around with email notification on both yahoo mail and outlook mail and it seems like I only get notification for my yahoo mail. However the email did mark as sent from your backend > easysocial > email activity where it is successfully sent the email to joomla email system and to your server.
I also notice that you are using PHP Mail as your mail transport agent which is known to have this kind of issue previously.
Unfortunately this is beyond our control but you can try to change it to SMTP and consult with your webhosting provider if they have some sort of email filtering that are specific for outlook/msn mail and see how it goes.
Hope these help.