Seen as a system of interactions, what does an email gateway need around it to work, and what are the two conditions for it to be effective?

It exchanges mail with the internal mail server and the internet, reads blacklist, malware-pattern and virus services, and moves suspicious mail into quarantine, while an admin maintains its rule set. It is effective only if all email traffic is routed through it, and if organisational processes exist for maintaining the rules and handling filtered emails.

* The gateway depends on outside pattern services and on an admin who maintains its rules and handles quarantine. *

The interaction view shows that the gateway is not a box you install and forget. It depends on outside services that keep its patterns current, and on a person: someone must maintain the rules and decide what happens to quarantined mail (release a false positive, confirm a real threat). Without those processes, either good mail disappears or the rules drift out of date.

Its job in one sentence: an email gateway with content control prevents unwanted content from being brought into, or out of, a system via email.

Go deeper:

• NCSC — Email security and anti-spoofing — practical guidance on protecting an organisation's email.
• NIST SP 800-45 Version 2 — Guidelines on Electronic Mail Security — securing mail servers, gateways and content filtering.