Hi @Chengjiang Jin,
Check the answer on this question: https://answers.atlassian.com/questions/16883503/how-to-block-spam-mails-from-entering-jira-service-desk-queues-from-email-channeling-
They are suggesting the use of CAPTCHA
Thanks,
Jared - Design Industries
The problem is that CAPTCHA was broken several years ago - this system of Answers is under constant attack and several accounts get past CAPTCHA here every day. If this user wasn't already using CAPTCHA, their system would be utterly unusable, as it is, all they can realistically do is delete spam accounts, and turn off the global ability to sign up.
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