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PMTA knowledge base›Block Global›Directive§ 3.3.1.57.1

dummy-smtp-ip

Global

Which address the fake receiver listens on
in the configuration this is a linedummy-smtp-ipinside the block Global
How far the black hole is visible

The address on which the node accepts connections for the fake receiver. It works in a pair with dummy-smtp-port: that one decides whether to listen at all, this one — where exactly.

The local address 127.0.0.1 is almost always the right choice. The fake receiver is needed for loading the node itself, and it is the same node that connects to it. By limiting the address to local, you make the black hole unreachable from outside.

An external address is rarely needed — when the load is created by another machine. Then the receiver is visible to the network, and it has to be protected by network means.

The main trap is leaving an external address. The fake receiver does not ask who came to it: it accepts everything and throws it away. Open to the network, it becomes a receiver any neighbor can use as a black hole — and on your node it will look like normal operation.
What the Hub does. It does not write it: the mechanism is not brought up on the nodes. If it appears, the address must be local — the node creates the load for itself.
Recommended values

A local address as the default way of thinking. The load is created by the same node, so listening on itself is enough.

External — only together with a firewall restriction. The receiver itself filters no one out.

On a separate node, not a production one. Even a local black hole on a production node is an open port that a typo in the configuration can send mail into.

The rule about the local address and the firewall is ours. From the manual comes only the purpose of the line: the address on which such connections are accepted.
Example configuration
# the black hole is visible only to the machine itself
dummy-smtp-ip 127.0.0.1dummy-smtp-port 2525
With a local address, connections from outside will not reach the receiver — neither accidentally nor deliberately.
yellow — — directive names, blue — blocks. They are highlighted the same way in the text of the card.
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Common mistakes and problems

what people get wrong and how it ends2 mistakes▶

Listening on all interfaces

our experience

What it costsThe black hole is open to the network. The receiver accepts everything from everyone, checking nothing — and on the node this looks like regular operation.
How to spot itIn the ss -ltn output the port shows 0.0.0.0.
How to do it rightLimit the address to local or close the port on the firewall.

Bringing the receiver up on a production node

our experience

What it costsA typo in any receiver’s server list will send real mail into the black hole — and it will be marked delivered, because the conversation went normally.
How to spot itMessages are counted as delivered but did not reach recipients.
How to do it rightKeep the mechanism on a separate node.
Compatibility
PowerMTA 6.0
described in the manual of this version · platform: any
The vendor does not state the first version for every directive — there is no note here. The name also appears in the 5.0 edition of the manual.
Reference data
Syntax
dummy-smtp-ip <address>
Values
an IPv4 or IPv6 address on which fake receiver connections are accepted
Applying
reload, no restart needed
Queue
will not touch what is already queued
Where to write it
Global
How many times
once in the block
Everything here comes from the manual, with the section.
Inheritance
A node-level line: it is written at the root of the file, not inside a block. It has no inheritance. It is picked up by pmta reload — together with dummy-smtp-port, which is what opens the port.
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Related directives

1 relation

Applies to

A fake receiver inside the node itselfOne of the eight lines configuring the fake receiver: this one is responsible for where it listens. § 3.3.1.57.1

How to check on the node
Which address the receiver is up on is visible with system tools — there are no node-level lines in the queue settings:
ss -ltn | grep 2525
grep -n dummy-smtp-ip /etc/pmta/config
The address 0.0.0.0 in the first command’s output means the receiver listens on all interfaces and is visible to the network. The Hub does not use this mechanism.
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What else to read

3 articles

◇The activation key is a prerequisite for installation, not a step after itWithout an activation key from SparkPost support the engine neither installs nor runs, and the license tier sets the outbound connection ceiling: on Standard everything above 200 is ignored.◇Cloud restrictionsClouds restrict outbound mail on port 25 until you ask them to lift the limit, and mailbox providers treat their address ranges with suspicion.◇How to turn a sending plan into hardwareTwo constants for sizing any node: about 2 KB of memory per queued recipient and spool space proportional to the total size of the messages.

In the Hub
Cost of a mistake
reversible: harms only while it is set
Scenarios
Capacity planning
Neighboring settings of the block Global:log-filelog-auto-rotationlog-rotatedummy-smtp-await-slotdummy-smtp-latencyAll 108 directives of the block →