could not receive external mails on james 3 at ubuntu

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http://serverfault.com – I have a domain registered say XXX, i have installed James mail server 3 on ubuntu server which is on windows cloud. After this i created A record which points to public ip of ubuntu cloud server and in MX settings in value against domain i had provided name of that ubunto cloud server. I can send mails to gmail from James Server3 but when i try sending mails from gmail to James server which is on Ubuntu server. Mails are delayed with error as follows.. I am not able to make out whether the problem is with 1) MX settings or 2) DNS not configured on Ubuntu or 3) I am missing some configurati (HowTos)