My external customers receive a blank email when they submit a comment to an issue via email.
jeti01.png shows what the customer receives. The message that only states "Re: Test 12" is the problem
jeti02.png shows what I see in Jira (looks good to me)
jeti03.png shows the email inbox for the assignee (looks good to me)
I am using JETI for the mail handler, the JETI post-function for the worklow transition states, and JETI notifications have been configured. Let me know what other information you need.