The Issue:
We have been receiving a lot of Postmaster Return Emails for email accounts not accepting the Lost Password & Registration auto-generated emails. The email service providers we are currently seeing this issue with are listed below. After looking at the error code for the reason why, below is the issue.

Problem Service Providers: (This list may be updated from time to time)

Why it's happening:
Hyperlink to excerpt below: 

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Causes for error code 4.4.7

When Exchange Online attempts to deliver a message, the destination email might be unable or unwilling to accept the message. This can result in a temporary 4_.x.x_ error code from the destination email server (instead of a permanent 5.x.x error code that indicates the message was rejected). Exchange Online repeatedly tries to deliver the message over 24 hours. Only after two days of unsuccessful delivery attempts does the recipient receive this NDR.

The possible causes of this error are:
- The destination email server is offline or unreachable.
- The server won't accept delivery of the message.
- A network problem is causing message delivery to time-out.

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1) Based on the information we have, solutions we have read and verifying that the Lost Password feature is delivering to other email service providers (i.e. gmail, yahoo, aol), you will need to reach out to your email service provider to resolve this issue. It could be as simple as them accepting all email from the domain on their server(s).
2) An alternative solution for this is to use another email service provider (i.e. gmail, yahoo, aol). If you have another you wish to use, please send a polite email to [email protected] for further assistance with changing your email address to assist you getting logged in.