The most common causes are:
– You don’t confirm opt-in to your mailing lists and don’t have any anti-bot mechanisms to prevent automation from adding recipients.
– Broken or missing bounce or engagement management.
– Email lists purchased from a 3rd party or use of any email appending services.
– Sending mail to very old customers or any address with whom you have had no interaction for > 2 years.
– Compromised accounts or services
– Infected computers or devices.
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