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Based on your description, I understand you have changed the SMTP address, but still in outlook it is showing old address.
This behavior is expected. The primary email address in Outlook will change in a few days. This delay is caused by the time it takes for the offline address book (OAB) to be updated in Exchange and the time it takes for Outlook to download the OAB. You could try to Create an Outlook profile - Microsoft Support to work around this behavior.
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