By default, Talend's tSendMail uses plain text. The following will enable tSendMail to send HTML formatted emails.
Go to the advanced settings of a tSendMail component, and change "MIME subtype from the 'text' MIME type" to HTML from "plain text".
Switch back to the Basic Settings tab. In the "Message" text box, add your HTML.
Now run your job and verify that tSendMail is in HTML:
https://www.talendforge.org/forum/viewtopic.php?id=3798
https://www.talendforge.org/forum/viewtopic.php?id=35580