Let's say a company has six different departments: Department A, Department B and so on. At each department, there is one person who serves as an "outward", or "external" contact person, so all communication between departments should go via this contact person. That is, if someone from Department A wants to get help with something from Department B, they contact Department B's "external" contact person, who will then take the matter to the right person within Department B. What do you call this person? External contact person? Outward contact person? Entry contact person? What?
Such a person is often called a "point of contact". If you want to emphasize that this POC is only for external relations, then I can't think of a better way of saying that except explicitly, e.g.:
He/she is our point of contact for external (or in this case perhaps "interdepartmental") relations.