Every email provider utilizes safeguards to protect user accounts and protect its users from potential malicious activity. If your email provider is preventing you from sending emails to certain addresses, it may be for one of the following reasons:

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1. The destination address is not valid or no longer active.
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2. The destination address is being blocked by the recipient’s email provider’s spam filters.
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3. The destination address is being blocked due to your sending email address or domain being flagged for spam activity.
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4. The destination address owner has manually blocked your email address.

In order to troubleshoot this issue, you should first make sure that your mailing list is up to date with valid and existing addresses. Additionally, confirm that your messaging content is compliant with your email provider’s guidelines and policies. Finally, make sure that you are not using multiple different email campaigns to contact the same individuals.