diff --git a/internal/smtp/user.go b/internal/smtp/user.go
index fd857dee..89d59ff2 100644
--- a/internal/smtp/user.go
+++ b/internal/smtp/user.go
@@ -251,6 +251,8 @@ func (su *smtpUser) Send(returnPath string, to []string, messageReader io.Reader
 		return
 	}
 
+	message.Sender.Address = pmapi.ConstructAddress(message.Sender.Address, addr.Email)
+
 	kr, err := su.client().KeyRingForAddressID(addr.ID)
 	if err != nil {
 		return
diff --git a/test/features/bridge/smtp/send/mixed_case.feature b/test/features/bridge/smtp/send/mixed_case.feature
new file mode 100644
index 00000000..747a6720
--- /dev/null
+++ b/test/features/bridge/smtp/send/mixed_case.feature
@@ -0,0 +1,38 @@
+Feature: SMTP sending with mixed case address
+  Background:
+    Given there is connected user "user"
+    And there is SMTP client logged in as "user"
+
+  Scenario: Mixed sender case in sender address
+    When SMTP client sends message
+      """
+      From: Bridge Test <[userAddress|capitalize]>
+      To: Internal Bridge <bridgetest@protonmail.com>
+
+      hello
+
+      """
+    Then SMTP response is "OK"
+    And mailbox "Sent" for "user" has messages
+      | from          | to                        | subject |
+      | [userAddress] | bridgetest@protonmail.com |         |
+    And message is sent with API call
+      """
+      {
+        "Message": {
+          "Subject": "",
+          "Sender": {
+            "Name": "Bridge Test"
+          },
+          "ToList": [
+            {
+              "Address": "bridgetest@protonmail.com",
+              "Name": "Internal Bridge"
+            }
+          ],
+          "CCList": [],
+          "BCCList": [],
+          "MIMEType": "text/plain"
+        }
+      }
+      """
diff --git a/test/mocks/smtp.go b/test/mocks/smtp.go
index 69529914..59efb8c6 100644
--- a/test/mocks/smtp.go
+++ b/test/mocks/smtp.go
@@ -75,6 +75,7 @@ func (c *SMTPClient) SendCommands(commands ...string) *SMTPResponse {
 
 	for _, command := range commands {
 		command = strings.ReplaceAll(command, "[userAddress]", c.address)
+		command = strings.ReplaceAll(command, "[userAddress|capitalize]", strings.Title(c.address))
 
 		tstart := time.Now()
 
diff --git a/unreleased.md b/unreleased.md
index 4bde40bf..0d2cf883 100644
--- a/unreleased.md
+++ b/unreleased.md
@@ -9,3 +9,6 @@ Changelog [format](http://keepachangelog.com/en/1.0.0/)
 ### Removed
 
 ### Changed
+
+### Fixed
+CSB-331 Fix sending error due to mixed case in sender address.