Hi Pat, 

This is how it is done: 

Delete old email addresses
Mail keeps a list of your previous recipients and uses it to suggest or 
complete addresses for you when you send email. You can delete obsolete or 
incorrect addresses from the list in Mail, and use the list to quickly see or 
add people to the Contacts app.

In Mail, choose Window > Previous Recipients.
Do any of the following:
Sort the list: Click a column header.
Search for a name: Enter the name in the search field. 
Remove a name from the list: Select one or more names, then click Remove From 
List.
Show a name in Contacts: Double-click any name that’s preceded by a card icon.
Add a name to Contacts: Select a name, then click Add to Contacts.



https://support.apple.com/en-au/guide/mail/delete-old-email-addresses-mlhlp1113/mac
 
<https://support.apple.com/en-au/guide/mail/delete-old-email-addresses-mlhlp1113/mac>

Regards, 

Philippe Chaperon

On 20 Aug 2018, at 6:06 pm, Pat <[email protected]> wrote:

I just wrote a letter to my sister; when I tried to put her new address on the 
letter, the app kept changing it back to an old, obsolete address.  The new 
address is in the Contact app and the old address has been removed. The new 
address wouldn’t stick even when I typed it in by hand. Is there a solution for 
this? I would rather not get a different mail app.

TIA,
Pat
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