Their email client can do it automatically. Basically, you just need to tell them, "Official emails will always have a big, green border around them."
Also, the number of people who fall for 419 scams is fairly low, just barely above the threshold of profitability. The reason people are shocked when they hear that anyone falls for such scams is that hardly anyone does. There is a hypothesis that 419 scams are designed to be obvious, because it helps in filtering potential victims: anyone who would be naive enough to reply is an easy target.
I think a broader problem is that most people are not just unaware of cryptography, but they use an email client that has no support for checking digital signatures. Webmail is by far the most popular email client type, but many popular webmail systems have no support for digital signatures at all, not even checking them for validity. It would be a lot easier to tell people to check for a digital signature if that meant looking for a border color, or a big gold star, or if hovering over/clicking on a link in an unsigned message displayed an annoying warning but no warnings were displayed in signed messages; sufficiently annoying warnings do help in making cryptosystems more effective in practice:
Also, the number of people who fall for 419 scams is fairly low, just barely above the threshold of profitability. The reason people are shocked when they hear that anyone falls for such scams is that hardly anyone does. There is a hypothesis that 419 scams are designed to be obvious, because it helps in filtering potential victims: anyone who would be naive enough to reply is an easy target.
I think a broader problem is that most people are not just unaware of cryptography, but they use an email client that has no support for checking digital signatures. Webmail is by far the most popular email client type, but many popular webmail systems have no support for digital signatures at all, not even checking them for validity. It would be a lot easier to tell people to check for a digital signature if that meant looking for a border color, or a big gold star, or if hovering over/clicking on a link in an unsigned message displayed an annoying warning but no warnings were displayed in signed messages; sufficiently annoying warnings do help in making cryptosystems more effective in practice:
https://bugzilla.mozilla.org/show_bug.cgi?id=460374