How about the logs, anything interesting in there ? You might want to turn on some debug logging for incoming mail handler, see this https://confluence.atlassian.com/display/JIRAKB/Diagnosing+Mail+and+Mail+Handlers+Issues (Advanced Assessment step 1)
This is a bug in JIRA default mail handler code, I have provided a fix in the bug report:
Before that fix is available in JIRA, you can use a work around: create all corresponding default reporter name in lowercase.
If you can't change your username, try my commercial plugin: Enhanced Mail Handler
It contains the fix and bunch of other fancy features!
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