The cause was the Junk Email setting in Outlook. It was set to block all countries in the International tab, including IO which is British Indian Ocean Territory. Group.io ends in ¡°io¡± so it was being treated as such.
Subject: Re: [GMF] Mail Delivery Issue w/ Comcast/Outlook
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I have a Comcast/Outlook mail delivery. While Comcast has a Spam folder, Outlook has a "Junk E-mail" folder which works like a Spam folder. Have your user check the Outlook "Junk E-mail" folder.