After a thorough search it seems to me that the Mail Queue page is not documented for JIRA Server. There is information on that page that can be explained for admins.

For example: 

1. Why are some mail items red?
2. When is a mail item moved to the error queue?
3. What does that little envelope icon mean?
4. What does "Anonymous mentioned you" mean? (https://confluence.atlassian.com/doc/the-mail-queue-151387.html is for Confluence)

The Mail Queue page is not available in Cloud but this is not included in the list of differences at 

https://confluence.atlassian.com/adminjiracloud/restricted-functions-in-jira-cloud-applications-776636921.html