Hi @Nicola Doyle ,
If you can send email to all recipients individually, there may be an issue with the distribution list's settings.
- Make sure you've allowed people outside your organization to send emails to the distribution group. The two toggles at the bottom of the Edit details pane should be set to On.
- External members don't receive email messages that are sent to a group they're a member of, and the senders don't receive non-delivery message about the email. Read External members don't receive email... for steps on how to fix this issue.
If you are still having problems, you can try deleting the distribution list and then re-creating a new one.
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