Here’s a list of things you can do to troubleshoot email delivery to recipients outside your organization:

1. Check the recipient’s email address for typos.

2. Check if your organization has an email authentication policy in place, such as SPF or DKIM, and ensure that it is properly configured.

3. Check the address against a block list or spam filter.

4. Check if your mail server is sending exceeding the daily quota of emails to a particular recipient.

5. Check for any security policies in place for sending emails to recipients outside of your organization.

6. Check if there’s a technical problem on the recipient’s end.

7. Check in case the recipient’s mailbox is full.