Why am I receiving a "delivery failed" notice for an email address that's not in my contacts list?

[TammyT41]

Every time I send our weekly email to all customers, I receive a "delivery failed" notice for an email address that's not in my list of contacts. How is this possible, and how can I stop it from happening?

[William_A]

Hello @TammyT41 ,

If any of your contacts are a distribution list or is set to auto-forward emails to others, or if one of your contacts is using a rewritten email to mask their true email, it can result in your email going to those not in your contacts.