Hi,
When did the issue occur?
Did you perform any actions before the issue occur?
What's the office product you are using to associate with mail recipient?
Please try to create a new user account to check if the issue persists.
Here is a similar thread which resolved the issue at last, please refer to NitramHa's reply in the last.
https://social.technet.microsoft.com/Forums/office/en-US/d387e8e9-428b-4add-acfb-c8cda9e102ce/send-to-gt-mail-recipient-outlook-2016-windows-7-issue
Bests,
Please remember to mark the replies as answers if they help.
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