For troubleshooting email delivery to a distribution list in Exchange, here are some best practices:

1. Check list size – Consider that mailboxes with a large number of recipients and large mailbox sizes can slow down email delivery.
2. Check list usage – Verify that the list usage is supported by limiting the number of messages, verifying that users have the permission to send to a list, and, if required, link to an approved sender.
3. Check list moderation settings – Verify that moderation is used if desired. It can help your organization control the email that is sent and received by other users.
4. Check outbound security controls – If you are using an outbound security control that scans your outbound emails, it may be blocked by these controls.
5. Check list properties – Make sure the list has the correct properties set, including whether or not the list is hidden and its membership is limited to approved senders.