Symptoms
You can’t find a user who was migrated to Exchange Online in the People Picker in the Exchange Admin Center (EAC). Additionally, the "Send as" and "Send on behalf" permissions can’t be added for legacy dedicated or on-premises mailboxes.
Cause
After mailboxes are moved from the on-premises environment to Exchange Online, they are represented as Remote Mailboxes. These remote mailboxes are not displayed in the EAC People Picker.
Workaround
This issue is being investigated by Microsoft. To work around this issue, run the following Remote PowerShell cmdlets to add these permissions:
-
"Send as" permission: Add-ADPermission
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"Send on behalf" permission: Set-Mailbox
