Creating incident via inbound email function doesn't work and this happened a few times before. It can be resolved by restarting the 'Mail Manager - Inbound Service' before. This time restart the service doesn't work.
"Test" button in the inbound mail setting page in console succeed. But Event viewer has an error message "username zfmm\helpdesk connect to server 10.0.1.21 failed". This error happens every minute because the poll frequency of inbound mail service is 1 min.
Helpdesk's email account size or mail quantity on mail server is too big. Inbound mail service cannot resolve the new incoming mails of service desk's account from mail server.
Archive the old mails of the account, that the inbound mail service used, on mail server, which is highly recommended to do on a regular basis such as half a year. The frequency depends on the size and quantity of email.