When someone replies to an email from ISN (or any provider) there is always a chance the content could be altered by the receiver's software. When a person hits reply the user's email program creates another "updated" copy of your original email (so it can show it as a reply). This "copy" is used to show the new receiving person what email they are replying to.
Here's the challenge...
When the software the original receiver is using creates the "reply content" for the email it may breakdown the email message. During this process it may not display correctly or it may be missing content. This is something 100% out of your control or ISN's control.
In ISN you can always visit Email -- View Sent Email to see how the message looked when it was sent. Please do not assume how an email looks in a reply is how the email went out.