I am now in the post office headquarters of Xiamen Zhongyu, a mailbox service provider. Send the same email content to the recipient with other email accounts to see if the other party can receive it. If it can be received, it is not a question of garbage filtering rules. You can try sending it from multiple different mailboxes or free mailboxes. If the other party doesn't receive it, then there may be something wrong with the content of the email. If you are a familiar customer, ask him directly if he blacklisted your email. \(^o^)/~