Delivery has failed to these recipients or groups:
'Sawitree K.' (Sawitree.chaicheatt@hoya.com)
The email address you specified couldn't be found or is invalid. It may be due to a bad entry in your Outlook or Outlook Web App recipient AutoComplete cache. Use the steps below to clear the entry from the cache:
1. Click New mail.
2. In the To field, start typing the recipient's name or email address until the recipient appears in the drop-down list.
3. Use the DOWN ARROW and UP ARROW keys to select the recipient, and then press the DELETE key.
Then resend your message – delete and retype the recipient's name or email address before sending it.
For more tips to help resolve this issue, see DSN 5.1.1 Errors in Exchange Online and Office 365.