diff --git a/detection-rules/abuse_docsend_unsolicited_reply-to.yml b/detection-rules/abuse_docsend_unsolicited_reply-to.yml
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+name: "Service Abuse: DocSend Share From an Unsolicited Reply-To Address"
+description: "DocSend shares which contain a reply-to address or domain that has not been previously observed by the recipient organization."
+type: "rule"
+severity: "high"
+source: |
+  type.inbound
+
+  // Legitimate DocSend sending infratructure
+  and sender.email.email == "no-reply@docsend.com"
+  and headers.auth_summary.spf.pass
+  and headers.auth_summary.dmarc.pass
+
+  // the message needs to have a reply-to address
+  and length(headers.reply_to) > 0
+
+  // reply-to email address has never been sent an email by the org
+  and not (
+    any(headers.reply_to, .email.email in $recipient_emails)
+    // if the reply-to email address is NOT in free_email_providers, check the domain in recipient_domains
+    or any(filter(headers.reply_to,
+                  // filter the list to only emails that are not in free_email_providers
+                  (
+                    .email.domain.domain not in $free_email_providers
+                    or .email.domain.root_domain not in $free_email_providers
+                  )
+           ),
+           .email.domain.domain in $recipient_domains
+    )
+  )
+  // reply-to address has never sent an email to the org
+  and not (
+    any(headers.reply_to, .email.email in $sender_emails)
+    // if the reply-to address is NOT in free_email_providers, check the domain in sender_domains
+    or any(filter(headers.reply_to,
+                  // filter the list to only emails that are not in free_email_providers
+                  (
+                    .email.domain.domain not in $free_email_providers
+                    or .email.domain.root_domain not in $free_email_providers
+                  )
+           ),
+           .email.domain.domain in $sender_domains
+    )
+  )
+tags:
+ - "Attack surface reduction"
+attack_types:
+  - "Credential Phishing"
+tactics_and_techniques:
+  - "Evasion"
+  - "Free file host"
+  - "Social engineering"
+detection_methods:
+  - "Content analysis"
+  - "Header analysis"
+  - "Sender analysis"
+id: "b377e64c-21bd-5040-86ec-534e545a42db"