One of my user is getting following error message.
"Delivery has failed to these recipients or groups:
recipientemailid@mycompanydomain.com
Your message wasn't delivered because the recipient's e-mail provider rejected it."
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He is sending a .pdf attachment to 3 recipients but getting the delivery failure report for all 3 users.
But when he sends it to one by one he is able to send it without any errors.
All other emails to same recipients are being sent properly even when sent to all 3 of them in the same email.
What could be the issue?