It seems SMTP Authentication is not enabled in your Email client and throwing an error “550 This message was classified as SPAM and may not be delivered”.
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Cause:
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SMTP Authentication is not enabled.
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Solution:
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If you are using Microsoft Outlook email client without having SMTP Authentication, then it will cause several points to be added to the spam score. Once we enabled SMTP Authentication in Outlook, this error might be resolved. You can enable SMTP authentication by using the following steps:
Step 1: Open Outlook
Step 2: Select the "Mail" tab.
Step 3: Double-click the Email account (e.g. mail.example.com)
Step 4: Select the "Servers" tab.
Step 5: Check the box next to "My Server Requires Authentication."
Step 6: Select the "Advanced" tab.
Step 7: In the Server Port Numbers section, modify "Outgoing mail (SMTP):" to "587".
Step 8: Click "OK
