Email Client unable to connect to server

Hi, I'm new to using Untangle and so far I have setup the Untangle server and an hMailServer on a separate windows box. Have created two Port Forward rules for SMTP and POP3 on the untangle server pointing to the fixed ip address of the email server. On email server I can resolve DNS and receive emails plus the diagnostic checks with the hMailServer are all ok. The dns record for my mail server resolves to the external ip address of the untangle server but I am unable to connect any email clients to the email server. When testing the accounts it tells me the mail server is not responding.

Any help would be most appreciated.