Article ID: 2830150 - View products that this article applies to.
When a user whose mailbox was migrated from the on-premises environment to Office 365 clicks Reply All in an email message, his or her email address is added to the To box of the message.
This happens for messages that the were sent to the user before his or her mailbox was migrated to Office 365. This doesn't happen for messages that were sent to the user after his or her mailbox was migrated to Office 365. This behavior is by design.
Have the user remove his or her email address from the To box when he or she replies to messages that were delivered to his or her mailbox before the mailbox was migrated to Office 365.
When a user mailbox is migrated from the on-premises environment to Office 365, a new mailbox is created for the user in Office 365. The new mailbox uses the domain name that your organization was given when the organization signed up for Office 365. For example, the new mailbox could use the domain name contoso.onmicrosoft.com. In this example, if the user's original email address before the migration was email@example.com, that user's mailbox in Office 365 will be firstname.lastname@example.org. Additionally, a proxy address of email@example.com is added to the firstname.lastname@example.org mailbox in Office 365. This is part of the process that makes sure that future messages that are sent to email@example.com are delivered to the Office 365 mailbox.
Messages that are sent to the user before the migration reference the user's original email address (in this example, firstname.lastname@example.org). Therefore, when the user replies to an old message, Outlook Web App can't find the current user in the list of recipients because Outlook Web App is looking for email@example.com. If Outlook Web App did find the email address, the address would be removed from the To box so that the sender didn't get a copy of his or her own reply. Instead, in this scenario, Outlook Web App finds and keeps the original email address as it was found in the list of recipients and adds it to the To box because it doesn't match the current email address of the sender.
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