Hi @Fang Dong ,
Welcome to our forum!
Through my extensive search, I didn't find any official documents which introduces the principle of spam filtering. It is recommended that you could select the trusted spam in the Junk folder and click on the Report Message option and choose Not Junk, which may have an impact on your spam filtering.
According to your description, I noticed that many emails you received have been moved to Junk folder, considering that you have set a higher protection level and blocked more messages, it is recommended that you could change the level of protection in the Junk Email Filter to check if there're any difference.
In addition, if you trust spam from the same sender or from the same domain, agree with yagmoth555, it is recommended that you could try to add the sender or domain names to the Safe Senders(Click Home > Delete group > Junk > Junk E-mail Options>Safe Senders>Add), so that email addresses and domain names in the Safe Senders List are never treated as junk email, regardless of the content of the message.
Hope the above help!
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