Problem with emailing comments to a ticket?

Problem with emailing comments to a ticket?

Emailing for the first time to my ticket will upload the comment correctly. Such as:


But, when I send another comment, the email address is remembered by my email program, so the email looks like this:

Notice the "saved" email in the CC. It is the same address. But, for some reason, this comment will not save. I get returned from Zoho instead:

We are unable to process this email.

WHY DID THIS HAPPEN?
We suspect the below reasons to be the cause of failure :

1. You might have mis-spelt the email address.
2. You might have added the email address in BCC

But, as you can see, neither of these applies. Why does this happen? How can I avoid this?