What term can be used when final correspondence is signed by someone other than the usual signatory?

The term that is properly used when final correspondence is signed by someone other than the usual signatory is "On behalf of." This phrase indicates that the person signing the document is doing so as a representative of another individual, rather than as the primary author or the intended signatory. This usage is especially important in formal communications where the authority and intention behind the signature must be clear, ensuring that the recipient understands that the signature is not from the individual normally associated with that role, but rather from a person acting in their stead.

The other terms do not convey the same level of clarity about the delegation or representation involved in such a signature. For example, "By" would generally indicate who physically signed the document and does not imply representation. "For" can suggest a similar idea but lacks the explicit acknowledgment of acting on someone else's authority that "On behalf of" provides. While "Delegated" hints at the concept of authority being passed, it isn't a standard phrase used in correspondence to indicate that one individual is signing for another. Therefore, "On behalf of" is the most appropriate choice in this context.