Inconsistent Behavior for Mail Handlers

Summary:

Creating issues through email in JIRA sometimes fail. Only error is

An error was encountered whilst handling messages from the mail server
"POP Mail Server" using the handler "class com.atlassian.jira.plugins.mail.handlers.RegexCommentHandler".

The error was:
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"Could not find a corresponding issue to comment and 'Create a new issue from each email' is not selected."
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The error and the email that caused the error are attached to this email.

If this is a recurring problem, you may wish to contact a system administrator to check the parameters of your mail fetching service:

Steps To Replicate:

Create 2 mail handlers

• The first should contain the following settings:

  Create a new issue from each email message
  POP Mail Server
  mailstore
   Support	 Task	
  Bulk: forward
  CC Assignee: false
  CC Watchers: false
  Create Users: false
  Default Reporter: <defaultreporter>
  Forward Email: <email-address>
  Notify Users: false
  Strip Quotes: false
  

• The second should contain the following settings:

  Add a comment before a specified marker or separator in the email body
  POP Mail Server
  mailstore
  Bulk: forward
  CC Assignee: true
  CC Watchers: false
  Create Users: false
  Default Reporter: <defaultreporter>
  Forward Email: <email-address>
  Notify Users: false
  Split Regex: /From: |> |-- /
  Strip Quotes: false
  

• Create an issue via email, sometimes it will fail and other times it will succeed