Viruses propagate as attachments.
Zip files obscure the contents.
Viruses propagating as attachments may be labelles .zip , masquerafding as zip files.

Mozilla Thunderbird is a popular email client, out in the world.
Emails embedded as forwarded in another email will be labelled as .emll files


So the NHS email system blocks .eml attachments and demands they be turned into .zip files.

Ha.