Yahoo is dropping incoming Email messages without user warning or consent

Yesterday while investigating a BUG in my business website, I found that Yahoo may drop an incoming email to a user just because they suspect this email to be spam, the mail gets dropped silently and the user will never get it in SPAM folder and will not get any warning or notice

How to reproduce

Send the following email from any Gmail account to any Yahoo account, the body should contain only the following text, no signatures, no additions or removals, add any subject

Dear,

We will also Review your Account and will
only contact you if we need more information

Trial Account
Username

Expected Behavior

The message should be delivered to the user inbox or in SPAM folder

Accrual Behavior

The message never reach the user, the sender gets an error reply saying “Message not allowed”

Conclusion

It seems a BUG in Yahoo Spam Filter system, instead of marking the suspected message as SPAM it is refused, imagine how much emails has been incorrectly dropped this way

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