The CRM Email will automatically add an Auto-Number at the tail of email subject.
Such like this: "Your case has been opened. CR000000001". We know that's for the tracking use, but some of the target Email server will block this "auto-number" emails, which means this kind of email cannot send out!
Not all of the Email server block these emails, but some of those do that. What's the solution?
If you go into your system settings, you can turn this off. However, by doing so you will not be able to track back to the originating object, inbound email will only be able to resolve to the contact/account based on the email address.
It allows the router to automatically relate the reply to the same object (e.g. Opportunity) that the outbound email was tied to. Otherwise, it needs to identify the object purely on the email address of the sender which is not as reliable.