checking step :
- To confirm is the SharePoint timer service problem or other issues, i have tested to send the email to other document library with email incoming enable. Is working , so we narrow down is not the SharePoint timer service issues.
- secondly need to check and see the email have send out from email server to our server or not , this can go to each web front end sever drive D:\inetpub\mailroot\Drop , you will see some item there. You can use notepad to open the xxx.eml item then you will see the sender from and to also other email detail . then this is confirm not email server issues.
resolution step :
- disable the incoming email and the document library then wait for few minute and enable it back.