I came across an obscure Dear Abby for Mac users.
Here is one of the posts about Mail.
Dear Mr. Tipman,
I am frustrated with Mail. I accidentally typed "Terry@macgroup.com" in the address bar. Now, every time I send mail to Terry, Mail suggests the bad address.
How do I fix this?
Signed – Frustrated in MacLand
In the Mail Window menu, click on Previous Recipients.
You will see the list of what mail remembers and also your Address Book entries with an icon on the left.
Use the search box or just scan the list to locate the bad entry. You can sort by Name, Email, or Last Used by clicking on the column headings.
Select the bad entry and click the Remove From List button.
There. All fixed. Feel better now?
Signed – Mr. Tipman