.eml Attachments rejected
.eml files have been used very successfully in the spreading of viruses. Some older email software in particular will blindly run scripts contained within .eml files - potentially allowing attackers free access to their system. We have never allowed the receipt of these files, as per the default configuration of cPanel servers globally.
Unfortunately some email software still attaches forwarded messages as .eml files, rather than simply including the forwarded content in the body of the email itself. These emails are bounced back to sender by our servers.
The simple solution is for the sender to configure their email software to forward emails "inline" rather than attaching .eml files. This also has the added benefit of being somewhat less confusing for the recipient than dealing with attachments that may or may not open correctly for them.
Instructions for Thunderbird users can be found at http://kb.mozillazine.org/Forwarded_messages_not_readable
Alternatively, the .eml attachment can be renamed or zipped up and re-sent to work around this issue.