Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound

I have some client who are trying to send an email to an external account but are getting the error below:

IMCEAEX-_o=NT5_ou=00000000000000000000000000000000_cn=1D6E7A5E84730A4194C83E0E36BFADEB@rcmcweb.org
Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'

Remote Server returned '550 5.1.11 RESOLVER.ADR.ExRecipNotFound; Recipient not found by Exchange Legacy encapsulated email address lookup'

I have try to clear the Autocomplete Cache and even went to Empty the clients complete List in the Options menu, I'm not sure what else that I can do if anyone has any idea on how to resolve this problem please let me know.