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.
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You use virus total I assume, I actually had to google the term and I have been involved with Thunderbird sup[port for more than 10 years. I think you might struggle to get such a thing introduced, based solely on a lack of interest or virus total users. But Virus total might be prepared to author an addon that does it.