Distribution list with external contacts

I have a distribution list on our Exchange server that contains internal mailboxes and external email address (contacts).
When I send email to this distribution list internally, the email gets delivered to both internal and external address.
The problem is when I send email from an outside account to the same distribution list, the email gets delivered to the internal mailboxes but NOT to the external email address (contacts)
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tigermattConnect With a Mentor Commented:
Are the external contacts set to only accept mail from authenticated users only? (it's a check box which needs to be unchecked in Exchange General tab > Delivery Restrictions button. Also make sure "Accept messages from everyone" is selected.

If the authenticated users only option is enabled, it would cause the problem you're seeing since when you send it internally, you're authenticated. However externally you're just one of the many addresses on the internet and aren't considered authenticated.

If that doesn't sort it, just send the message via OWA or configure outlook with RPC over HTTPS and do it that way.
Forced accept.

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