fix(smtp): add STARTTLS support for port 587

- Add STARTTLS handshake after EHLO for port 587
- Upgrade socket to SSL after STARTTLS command
- Perform second EHLO over encrypted connection
- Resolves authentication issues with Hetzner and other SMTP providers
- Fixes 'Must issue a STARTTLS command first' error

Closes #113

src/smtp.lua

@@ -237,6 +237,33 @@ function SMTP:send_email(to_address, subject, message, from_name)
         return cleanup_and_fail("EHLO failed: " .. response)
     end
 
+    -- STARTTLS hinzufügen für Port 587
+    if self.port == 587 and self.use_ssl then
+        -- STARTTLS command
+        local success, response = self:send_command(sock, "STARTTLS", 220)
+        if not success then
+            return cleanup_and_fail("STARTTLS failed: " .. response)
+        end
+
+        -- Upgrade connection to SSL
+        local ssl_sock, err = self.ssl_compat:wrap_socket(sock, {
+            mode = "client",
+            protocol = "tlsv1_2"
+        })
+
+        if not ssl_sock then
+            return cleanup_and_fail("SSL upgrade failed: " .. err)
+        end
+
+        sock = ssl_sock
+
+        -- EHLO again over encrypted connection
+        local success, response = self:send_command(sock, "EHLO furt-lua", 250)
+        if not success then
+            return cleanup_and_fail("EHLO after STARTTLS failed: " .. response)
+        end
+    end
+
     -- AUTH LOGIN
     local success, response = self:send_command(sock, "AUTH LOGIN", 334)
     if not success then