Hi - We have Informix 4GL running in AIX 2.7 environment. We wanted to sending an HTML email from this environment so that proper tables and styles are be opted for user experience. Can you please advise on this? 

Check that you can send html files from the AIX shell first, and if that's good, then you should be able to create an appropriate file from 4GL (like you do now with plain text emails) and send it that way.  If you can send plain text emails with sendmail, then I expect that you can send HTML emails.  The key is to put in the correct header info and formatting into the text file including Content-Type: text/html.

Are you sure that you are using AIX 2.7?  Run "oslevel" to confirm.