I purchased another domain last week and had the ISP change records so the new domain pointed to my email servers outside IP. I can send email as firstname.lastname@example.org to email@example.com and have that message delivered to the proper email addresses company wide.
The one thing that I cannot do is "send as" firstname.lastname@example.org to any outside email address.
The undeliverable message I receive is:
You do not have permission to send to this recipient. For assistance, contact your system administrator.