Secure email gateway
A filtering service placed in front of an organisation's mailboxes, usually by pointing its MX records at the gateway, which scans mail before passing it on.
What it means
Some organisations receive mail through a gateway from a security vendor (Mimecast, Proofpoint, Barracuda, Cisco and others) rather than straight into Microsoft 365 or Google Workspace. Microsoft documents the setup as the MX record pointing to third-party spam filtering, and strongly recommends Enhanced Filtering for Connectors or a filtering bypass in that case, warning that skipping this step results in misclassified inbound mail.
For senders, a gateway is another filter with its own reputation data, rules and error codes. Barracuda's, for example, uses the Barracuda Reputation database of IP addresses, built from spam traps and other systems. A message can pass Microsoft's checks and still be stopped at the gateway.
The gateway's name in the recipient's MX records shows which filter you face.
Related terms
On this site
Sources
What this definition rests on. The wording is ours; the facts are theirs.
- Microsoft Learn: Manage mail flow using a third-party cloud servicelearn.microsoft.com
- Barracuda Campus: Barracuda Reputation and email categorizationcampus.barracuda.com
All 432 terms in the glossaryThe wiki: blocklists and bounce codes