Thanks Philippe,
I too was also looking for a way to do this.

Jewels
> On 20 Aug 2018, at 6:31 PM, Philippe Chaperon <[email protected]> wrote:
> 
> 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] 
> <mailto:[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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