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

lib/postal/smtp_server/server.rb

@@ -49,7 +49,7 @@ module Postal
         if ENV['SERVER_FD']
           @server = TCPServer.for_fd(ENV['SERVER_FD'].to_i)
         else
-          @server = TCPServer.open('::', Postal.config.smtp_server.port)
+          @server = TCPServer.open(Postal.config.smtp_server.bind_address, Postal.config.smtp_server.port)
         end
         @server.autoclose = false
         @server.close_on_exec = false
@@ -62,7 +62,7 @@ module Postal
           @server.setsockopt(Socket::SOL_TCP, Socket::TCP_KEEPCNT, 5)
         end
         ENV['SERVER_FD'] = @server.to_i.to_s
-        logger.info "Listening on port #{Postal.config.smtp_server.port}"
+        logger.info "Listening on  #{Postal.config.smtp_server.bind_address}:#{Postal.config.smtp_server.port}"
       end
 
       def unlisten