feat(smtp_server): allow bind address to be configured

closes #1166

config/postal.defaults.yml

@@ -72,6 +72,7 @@ workers:
 
 smtp_server:
   port: 25
+  bind_address: '::'
   tls_enabled: false
   tls_certificate_path: # Defaults to config/smtp.cert
   tls_private_key_path: # Defaults to config/smtp.key