problem with lookup

[jmiguel77]

Hi
I created a simple mail server with this command

docker run -d --name mailserver -p 25:25 bytemark/smtp

Then tried to send a message to my personal gmail account and got this error

  240 delivering 1kkUzM-00003r-F0
  240 R: dnslookup for jmiguel77@gmail.com
  240 LOG: MAIN
  240   == [myaccount]@gmail.com R=dnslookup defer (-1): host lookup did not complete

How can i fix this ?