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This is Justin with Microsoft Office 365 SharePoint Support. Thanks for posting your question.
It would appear that when customers use custom domains sometimes the email does not reach the recipient. This could be for a variety of reasons such as something at the registrar, spam or threat removal at the email provider level, junk or various other filters that are set up, and other factors. Does this happen when another email is used as mch mentioned? Please let us know.
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