MailChannels Outbound Filtering: SMTP relay overview

MailChannels Outbound Filtering is an SMTP relay service that identifies spammers within your network so you can protect your email reputation and keep delivery reliable for every user. Unlike conventional email relay services, Outbound Filtering accepts 100% of your outbound email — including spam — and then filters it. This approach means you never lose visibility into what your senders are doing, even when something goes wrong.

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How it works

Outbound Filtering works like a credit score for email senders. Every user, application, or script that sends mail through MailChannels builds a reputation score over time. The system tracks each sender individually, so a compromised WordPress plugin or a single hijacked user account can be isolated and shut down without affecting everyone else on your server. When you point your mail server at MailChannels as a smart host, every outbound message passes through the filter before it reaches the recipient. MailChannels delivers the message, removes spam, and manages IP blocklisting on your behalf.

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Compromised account detection

Spammers constantly search for new resources to exploit — through phishing attacks, malware, and web application vulnerabilities. Because you cannot reliably prevent every compromise, Outbound Filtering watches the email these compromised accounts generate and shuts them down automatically. The system uses several techniques to identify compromised senders:

• Spam signature matching — Each message is checked against a real-time database of known spam signatures. A sudden uptick in matched messages can trigger rate-limiting or blocking for that sender.
• Volume and behavioral statistics — MailChannels tracks dozens of per-minute statistics for every sender: message volume, suspicious content patterns, recipient validation failures, and more. Unusual trends surface spammers before significant damage occurs.
• External reputation databases — The system consults domain and IP reputation services to flag messages connected to known spam operations.
• Receiver feedback analysis — MailChannels monitors responses from recipient mail servers. Negative feedback can identify a compromised sender even before other signals appear.

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Notifications

When Outbound Filtering detects a compromised sender, the message is rejected automatically during the SMTP transaction. The sender receives a non-delivery report containing an error code that can be used for troubleshooting. If you have configured Monitors, a notification is also sent to alert you — so you can investigate and remediate the root cause, for example by disabling a user account or deactivating a vulnerable plugin. Notifications can be delivered as human-readable emails or as webhook calls.

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Multi-password support

You can create multiple SMTP passwords for a single account — one per server if you choose. If a server is compromised, you retire its password with a single click without disrupting any other servers.

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Multi-user support

Outbound Filtering supports unlimited web console logins. You can assign regular or administrator access levels, making it straightforward to give your support team read access while reserving administrative controls for senior staff.

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Email management features

• Log Search — Every delivery attempt is logged. You can search months of logs in seconds through the Host Console to diagnose delivery problems or answer questions from your users.
• Insights — When a message is rejected, Outbound Filtering generates a non-delivery report (NDR) with a link to an Insights page. Users can review their own sending history and often resolve delivery issues without contacting your support team.

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Domain Lockdown

Domain Lockdown is an optional but strongly recommended DNS-based security feature. Adding a _mailchannels TXT record to your DNS ties your domain to your MailChannels account, preventing other MailChannels customers from sending email that appears to come from your domain. See Domain Lockdown for setup instructions.

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What to do next