Summary

Jira is not processing any incoming emails - issues are not created.  This issue applies to mail handlers that are using the POP Mail Server(default mail server).

Steps to Reproduce

1. Create a mail handler that is using the "POP Mail Server"
2. "Edit" the "POP Mail Server" and click "Test Connection"
3. Send a test email

Expected Results

• Tests are successful
• Issues are created from emails

Actual Results

• Incoming email doesn't create a new issue
• Jira application logs are showing this error:

  2018-01-17 11:43:24.220

  WARN

  com.atlassian.mail.incoming.mailfetcherservice

  mail_handler_name[10301]: com.sun.mail.util.MailConnectException: Couldn't connect to host, port: mailstore, 110; timeout 10000 while connecting to host 'mailstore' as user 'jira@instance_name.atlassian.net' via protocol 'pop3, caused by: java.net.UnknownHostException: mailstore

Notes

• The default mail server hostname is "mailstore"
• When testing the mail server connection, an error is thrown: 

  Unfortunately no connection was possible. Review the errors below and rectify:
  
  UnknownHostException: mailstore
  

• In some(but not all) cases, the email sender is getting a bounce message - "550 5.1.1 Mailbox does not exist"

Workaround

• No known workarounds