Did you notice when you send an email in Apple Mail or in Outlook, and you start to type an email address, a list of email addresses appears. Some of those names are not in your address book and some are outdated email addresses. You can delete the autofill email addresses.
If you are using Outlook:
Create a new email message.
In the To field, start to enter a name or address
Press the Down Arrow key to highlight the entry you want to delete from the list.
Select Delete (the X to the right of the contact name)
If you are using Apple Mail:
Click on Window > Previous recipients
A list of addresses will appear, highlight an address and click remove