After importing messages into Outlook, they may display the wrong email account. You can use a macro to change the account shown in the Email Account field.
When you use the Resend button in an NDR to resend a message, the incorrect email address may be visible on the received message.
How to send an email message to all addresses assigned to a Microsoft Outlook contact.
An Office 365 user was trying to make Outlook use his vanity domain as the From address instead of the Office 365 default domain (onmicrosoft.com). This is not something that is controlled from Outlook, its set on the server. While you can have multiple addresses assigned to your mailbox (and receive mail sent to those
Microsoft Outlook (every version) has just three address fields. Some users need more fields: Email addresses are used for more than sending emails. They are also used by the social connector to bring together all information about a contact. Why are contact's limited to 3 addresses? There should be room for an unlimited number of
Add additional SMTP addresses to Exchange mailboxes, individually or using a recipients email address policy.