diff --git a/test/features/bridge/smtp/send/send_append.feature b/test/features/bridge/smtp/send/send_append.feature
new file mode 100644
index 00000000..ea1058d9
--- /dev/null
+++ b/test/features/bridge/smtp/send/send_append.feature
@@ -0,0 +1,50 @@
+Feature: SMTP sending with APPENDing to Sent
+  Background:
+    Given there is connected user "user"
+    And there is IMAP client logged in as "user"
+    And there is IMAP client selected in "Sent"
+    And there is SMTP client logged in as "user"
+
+  Scenario: Send message and append to Sent
+    # First do sending.
+    When SMTP client sends message
+      """
+      To: Internal Bridge <bridgetest@protonmail.com>
+      Subject: Manual send and append
+      Message-ID: bridgemessage42
+
+      hello
+
+      """
+    Then SMTP response is "OK"
+    And mailbox "Sent" for "user" has 1 messages
+    And mailbox "Sent" for "user" has messages
+      | externalid      | from          | to                        | subject                |
+      | bridgemessage42 | [userAddress] | bridgetest@protonmail.com | Manual send and append |
+    And message is sent with API call:
+      """
+      {
+        "Message": {
+          "Subject": "Manual send and append",
+          "ExternalID": "bridgemessage42"
+        }
+      }
+      """
+
+    # Then simulate manual append to Sent mailbox - message should be detected as a duplicate.
+    When IMAP client imports message to "Sent"
+      """
+      To: Internal Bridge <bridgetest@protonmail.com>
+      Subject: Manual send and append
+      Message-ID: bridgemessage42
+
+      hello
+
+      """
+    Then IMAP response is "OK"
+    And mailbox "Sent" for "user" has 1 messages
+
+    # Check that the external ID was not lost in the process.
+    When IMAP client sends command "FETCH 1 body.peek[header]"
+    Then IMAP response is "OK"
+    And IMAP response contains "bridgemessage42"
diff --git a/test/store_checks_test.go b/test/store_checks_test.go
index 435d6f34..3a5dd9b4 100644
--- a/test/store_checks_test.go
+++ b/test/store_checks_test.go
@@ -211,6 +211,10 @@ func messagesContainsMessageRow(account *accounts.TestAccount, allMessages []int
 				if message.ID != id {
 					matches = false
 				}
+			case "externalid":
+				if message.ExternalID != cell.Value {
+					matches = false
+				}
 			case "from": //nolint[goconst]
 				address := ctx.EnsureAddress(account.Username(), cell.Value)
 				if !areAddressesSame(message.Sender.Address, address) {