The Adagio email covers are sent as plain text, not HTML or RTF. Unfortunately, this means that little to no formatting is available. Your customer should know that it is not possible to send malware through plain text emails (links are another issue). Their best bet is to guarantee to their customers that the emails will come from a specific email address and perhaps contain some constant text in the subject.

When we were last working on the cover sheets, we had trouble getting HTML to work reliably.

The suggestion is on file to revisit this at some point.