How do fake letters from a trusted addressee end up in the mail?

Question

gmail

Hello. Stolunilsya with such a problem. There is a corporate mail domain on Yandex. Unexpectedly, once a week, fake letters began to come to my colleagues’ mail, supposedly from this mail (listed in the senders) with a link to a file from Google Drive, which I allegedly shared. In fact, the link leads to some kind of viral zip archive.
Where to look for the root of the problem and how to solve it?

Answer 1

There is no sender validation in "classic" email. To protect the sender's address from forgery, the DMARC protocol is used, you can read about how to properly implement DMARC to protect the corporate domain here .