In electronic mail systems, what do we call the component that connects two or more dissimilar email systems and transfers messages among them?

Introduction / Context:
Early and even modern email infrastructures often involve different systems and protocols (e.g., SMTP, X.400, proprietary groupware). A special component is needed to interconnect these heterogeneous worlds and relay messages between them.

Given Data / Assumptions:

• We need a function that interlinks different mail systems.
• It must accept messages in one format and deliver them in another when necessary.
• It performs routing/translation between domains or protocols.

Concept / Approach:
A mail gateway is designed for inter-system mail exchange and format translation. It differs from user-facing software (user agents) and link-layer devices (bridges) that operate at different layers/concerns.

Step-by-Step Solution:
1) Identify the need: interconnect heterogeneous email systems.2) Recognize the specialized role: accept, translate (if needed), and forward messages.3) The correct term is “mail gateway.”

Verification / Alternative check:
Standard email architectures show gateways between SMTP-based systems and other mail domains or legacy protocols.

Why Other Options Are Wrong:
Gateways (generic): Too broad; the precise term here is mail gateway.

Bridges: Operate at data-link level to forward frames; not an email-layer function.
User Agent: Client software (e.g., Outlook) for composing/reading mail, not inter-system transport.
Message spooler: Queueing component, not necessarily cross-system translator.

Common Pitfalls:
Confusing a general network gateway with the specific mail gateway function, or mixing up user agents and transport relays.

Final Answer:
mail gateway