When an organisation moves their email systems to Microsoft 365 Exchange Online, users may experience issues sending emails to recipients they had previously.
It happens especially when your former email system used Outlook to send messages - and you are attempting to forward or reply to emails that have been migrated to Microsoft 365.
In short, Outlook is retaining a memory of contacts and how it used to send message to them before - which Microsoft 365 does not know how to deliver. To fix this, clear the Auto-Complete list in Outlook.
Clear all entries from the Auto-Complete List
|Note: The Auto-Complete List is the pull-down display of contacts that appears as you type an email address in a new message window. Clearing this list will regenerate a new list over time.|
- Open Outlook. Then click File, Options and then Mail.
- Click Empty Auto-Complete List.
Click Yes at the warning.