If I understand you correctly, you want to send mail in HTML format using linux sendmail command. This code is working on Unix. Please give it a try.
echo "From: [email protected]
To: [email protected]
MIME-Version: 1.0
Content-Type: multipart/alternative;
boundary='PAA08673.1018277622/server.xyz.com'
Subject: Test HTML e-mail.
This is a MIME-encapsulated message
--PAA08673.1018277622/server.xyz.com
Content-Type: text/html
<html>
<head>
<title>HTML E-mail</title>
</head>
<body>
<a href='http://www.google.com'>Click Here</a>
</body>
</html>
--PAA08673.1018277622/server.xyz.com
" | sendmail -t
For the sendmail configuration details, please refer to this link. Hope this helps.