On May 13th, 2014 WHMCS released new upgrade to version 5.3.7. And we have upgraded it on lots of sites during the past month. But there was bug in the update which made it send blank emails. To fix this
- Login to WHMCS as administrator
- Click on Setup -> General Settings -> Mail Tab
- In drop down Mail Encoding change the value from 8bit to quoted-printable
- Now check if the mails are working properly and change it back to 8bit.