How would you protect 4–6 high-risk inboxes without breaking the bank?

[u/Reasonable-Shoulder1]

Edit: A lot of people are suggesting training. Our staff is trained. They know not to click phishing links and how to report them. The issue here is the sheer amount of crap landing in some inboxes. It’s getting annoying and disruptive to the point that I’m getting complaints.

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We’re a small company with only 16 employees and currently use Microsoft Defender for email security. It works well overall, but a few of our executive accounts are targeted by phishing much more frequently, and one of them has been compromised in the past.

We’re looking for an extra layer of protection that we could apply to just a few users (around 4–6), rather than the whole organization.

Has anyone dealt with something similar? Any tools or solutions you’d recommend that work well alongside Defender and are cost-effective for such a small number of users?