If I receive a file attachment, I would like to transfer it directly to Virustotal.com, either via Thunderbird's API or by right-clicking -> Send to Virustoal.
The file could be cached in a "to be examined" folder on the hard drive and then transferred to Virustotal.
In principle, everyone could use their own account API key for Virustotal. API reference https://docs.virustotal.com/reference/overview
If Process Explorer can send all processes to Virustotal, then you could certainly also send email attachments to Virustotal.