There isn't an .exe, it's an in-memory mail queue that sends data via your mail server using SMTP.
You specify a mail server in your configuration, here's how:
If you have problems with your mail from Jira, there are some pages which may help
https://confluence.atlassian.com/display/JIRA041/Troubleshoot+Mail+Notification+Problems
https://confluence.atlassian.com/display/JIRAKB/Mail+Queue+Never+Flushed+Automatically
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