MCEAEX '550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found' External Email

Greetings Everyone,

    When users try to send to two email addresses that are external they immediately get:

    IMCEAEX-_o=NT5_ou=00000000000000000000000000000000_cn=BC5E22A49F8C884095E915F3F5899A86@domain.com
    Remote Server returned '550 5.1.1 RESOLVER.ADR.ExRecipNotFound; not found'

    I have read the other similar questions but they pertain to internal mail and X500.

 
    All other email flows correctly to the same domain.

    Thank you.