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Got a Failed Delivery notice for an email you didn't send

When a sent email is rejected by an email provider, an automated Failed Delivery notification is sent to the email address included in the "From" field of the original message. If you didn't originate the email that prompted the notice, then someone else put your email address in the "From" field of their outgoing email.

This act is known as "email spoofing" and is often used by spammers to disguise their identity so that their recipients are more likely to open their email. While it's not possible to prevent someone from email spoofing, it only means they know or guessed your email address and don't necessarily have access to your account.