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Some Permissive SMTP Servers (like AOL) will send an "OK" response to the RCPT command, even if the user doesn't exist. That's called a "false positive". That's because the email was *probably* delivered, but we are not 100% sure.
In order to avoid false positives, you should turn the safemode on:
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÷Go to the Tools | Configuration | Servers and check the "Save Mode" box.
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Now, all "false positives" will have the status "Possible Success".