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Repair common Windows errors and clear accumulated junk for a smoother, more stable PC - no reinstall needed.Free scan · no reinstallA directory harvest attack (DHA) is an attempt to discover valid email addresses at a domain by sending guessed recipient addresses to its mail system and observing how it responds. Attackers can use the addresses that appear valid to build lists for spam. A DHA exploits how a mail system handles recipient checks; it does not, by itself, mean an employee mailbox was breached.
How a directory harvest attack works
Email servers use SMTP to exchange messages. During delivery, a sending server can issue a RCPT TO command naming a recipient. The receiving system responds, and differences in its responses may let a sender infer whether that address is accepted.
- Guess addresses: An attacker tries likely recipient names at a target domain, such as common names or predictable address patterns.
- Observe responses: The attacker compares the mail system’s replies to determine which guesses appear to be valid recipients.
- Build a list: Addresses that appear valid can be retained and used for unsolicited email or spam.
This is a form of address discovery, not necessarily account compromise: the attacker is testing the mail system’s recipient-validation behavior rather than logging in to each mailbox.
Why disabling VRFY may not be enough
SMTP includes the VRFY and EXPN commands, which can expose information about users or mailing lists. RFC 5321 identifies security concerns with these commands and says a site may disable them or limit their use to authenticated requestors. But the standard also warns that RCPT can reveal similar address-validity information in many cases, depending on when the system checks recipients. Disabling VRFY and EXPN alone therefore does not necessarily prevent a DHA. RFC 5321, October 2008
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How mail administrators can reduce the risk
Defenses focus on recipient validation and limiting what a remote sender can learn or do when trying invalid recipients. The right behavior depends on the mail system and its delivery workflow.
Check recipients during the SMTP conversation
A gateway can validate recipient addresses while the sender is connected and apply a limit to invalid-recipient attempts. Cisco’s AsyncOS 13.5.1 guide describes dropping a connection when its configured threshold is reached; under that threshold behavior, the envelope sender does not receive a bounce for an invalid recipient. A limit can constrain repeated guessing, but administrators need to choose a policy that fits their environment and legitimate delivery patterns. Cisco AsyncOS 13.5.1 guide
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Validate recipients after accepting the message
Another approach is to accept the message during SMTP and check recipients later in a work queue. Cisco says this can prevent a sender from learning recipient validity during the SMTP conversation. The operational trade-off is that an invalid recipient may still cause a later bounce to the envelope sender.
Set a threshold and connection policy
Administrators should define what happens after a sender reaches the invalid-recipient limit—for example, whether the gateway rejects further recipients, defers them, or disconnects. Thresholds and defaults are product-specific, not universal recommendations. Cisco’s AsyncOS 13.5.1 guide lists a default of 25 invalid recipients per hour for a public listener, while its private-listener default is unlimited; those figures apply to that product and version, not to mail servers generally.
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Include recipient validation in relay security
Australian Signals Directorate and Australian Cyber Security Centre guidance includes preventing directory harvesting among mail-relay security actions and says inbound relays should be able to validate recipient addresses before accepting delivery. ASD/ACSC gateway guidance
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What to remember
- A DHA tests guessed email recipients against a mail system’s behavior to identify addresses that appear valid.
- SMTP responses to
RCPT TOcan reveal recipient validity even whenVRFYandEXPNare restricted. - Recipient validation, invalid-recipient thresholds, and carefully chosen connection handling can reduce exposure; the specific settings and trade-offs vary by mail platform.
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