SMTP Spam Gateway: Filter Before the Mail Server
An SMTP spam gateway sits in front of Exchange or any MTA. How that differs from a mailbox plugin, and when Hexamail Guard is the on-premise version of that pattern.
Gateway vs “antivirus on the mailbox”
A spam gateway (email security gateway, anti-spam gateway — same buying question) is an SMTP listener. Other servers deliver to it; it delivers clean mail to Exchange, Hexamail Server, Postfix. Malware never lands in the store. Outlook plugins cannot make that claim.
Cloud gateways change your MX to Mimecast/Proofpoint. On-prem gateways change your MX or an internal smarthost to a box you run. Hub: on-premise spam filter.
Architecture
Internet → Guard (port 25) → your mail server. Outbound can reverse the path so Guard learns allowlists from real sent mail. Optional: collect POP/IMAP from an ISP, scan, then inject.
Hexamail Guard as the on-prem gateway
Any SMTP server behind it. SPF, DKIM, DNSBL, SURBL, Bayes, greylist, quarantine, AV. Windows or Linux. If you wanted the cloud MX version of this diagram, that is Mimecast — see cloud vs on-prem.
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