Sorry about the confrontational heading - my real question is why is digital signing so rare in general and particularly in emails from companies, especially security and tech companies?
But I do have to point out that the confirmation email for joining this forum was unsigned, as was an advertising email from Comodo trying to sell the advantages of signing emails! I did not notice if the process emails for obtaining the free Comodo email signing certificate were signed (but I suspect not!). ???
There would be a huge benefit if digital signing by companies became more widespread. Security providers (like Comodo) could provide automatic filters that would maintain lists of companies known to sign their emails and automatically block any email purporting to come from them but unsigned or incorrectly signed. We could stop this form of phishing attack (and also put a big dent in spam volumes) pretty much overnight if there was the will on the part of the industry and regulators.