I was looking at this information from the following link and wanted to know what exactly does the JIRA update using this example and is there a way to create an Issue via email to have an ID in a custom field?
IF you are asking whether we can create an issue or a comment from an email- then of-course we can. But, if you are looking for more complex things like defining something (in the email) for a custom field, then it is not possible. You can refer the previous answers for more details:
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