Remove SMTP check on port 25.

Outbound traffic on port 25 is restricted by AWS. Removing the
restriction will raise the cost of the project.

src/smtp.py

@@ -23,7 +23,11 @@ def check_smtp(port):
 
 def handler(event, context):  # pylint: disable=unused-argument
     """Lambda event handler."""
-    for port in [25, 587]:
+    # Port 25 is blocked for Lambda functions. To remove the restriction, the
+    # functions need to execute inside a VPN with a NAT gateway. This will
+    # raise the cost. For now assume that if the submission port is working,
+    # the smtp port is working too (same gateway, same host, same processes).
+    for port in [587]:
         success, message = check_smtp(port)
         print(message)
         if not success: