You can use mime mail and force send the body of the message in text format. Here is an excerpt from the readme file of the module:
, . mimemail():
$sender - a user object, text email address or an array with name, mail
$recipient - a user object, text email address or an array with name, mail
$subject - subject line
$body - body text in HTML format
$plaintext - boolean, whether to send messages in plaintext-only (default FALSE)
$headers - a keyed array with headers (optional)
$text - plaintext portion of a multipart e-mail (optional)
$attachments - array of arrays with the file path, MIME type (optional)
$mailkey - message identifier
return - an array containing the MIME encoded message
- $plaintext TRUE. .