Hi Andrew,
Sorry for the inconvenience.
To troubleshoot the issue, can you please provide the following information?
- Does the issue occur in OWA (Outlook Web App)?
- Do all the Exchange Online users in your organization encounter the same issue?
- Are there any differences or special characters among these recipients, for example, the “+” string in the email address?
At the same time, please capture a screenshot of the errors and upload it here. To do that, click “Use rich formatting” in the reply form, then use the button in the red mark below to upload it:
Image may be NSFW.
Clik here to view.
