Hi,
The Autocomplete list contains SMTP addresses, LegacyExchangeDN entries, and display names for people to whom you have sent mail previously.
In your case, we can clear the old autocomplete list and recreate new entries. To do this, please click the “X” in the list.

We can also go to File > Options > Mail > Send Message section. Click Empty Autocomplete List. This will delete all entries.
Hope this is helpful to you.
Regards,
Perry
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