It is a technique that basically consists of falsifying the headers of an email to bypass the recipient's spam filters. In this falsification, the recipient's own email is put in the email from which said email comes.
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This way, since most spam filters do not detect it, the email enters as if it were a normal email.
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This does not mean that spam is being sent from your emails to other users, but quite the opposite, they are sending spam to you taking advantage of this email bug.
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There is little that can be done about this nowadays. However, our company has taken all possible measures to solve it, although they are not 100% reliable.