Did you use an email address book?
Were the recipients' email addresses in the CC line?
If the email addresses are not published as an email address book for use in the mail merge, or if the email addresses are not on the To line when using mail merge, then they will be visible to all recipients.
Have you ever used Mail Merge before, or were you using it for the first time?
How was the email list linked to the recipient line?
Do you mean each recipient was able to see all the other recipients' email addresses?
I'm still not sure how you merged the recipients' email list with Outlook.
Did you use the Mail Merge Wizard?
The correct way to create a mail merge is:
On the Mailings tab, click Start Mail Merge, and then click Step by Step Mail Merge Wizard.
Was the recipients list also newly created in Office365, or did it originate elsewhere?
Ok, I would suggest trying another trial run of just a few email addresses to test the system again and see if it works. If it still shows all recipients' email addresses, then try another test by adding a few contacts from the People tab instead.
Ok, let me know how it goes, or if you need further assistance.
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