Read more about bulk operations here: https://confluence.atlassian.com/jiracorecloud/editing-multiple-issues-at-the-same-time-902498099.html
Sorry I can't help you here.
There is no way to bulk edit with Mail handler without sending a lot of emails.
Secondly, you would have to have some automation to transition the issues when the email is sent(and by the way, you have to make sure the transition is possible).
Finally, I hope you realize that JIRA would likely timeout your mail handler if it sees way too many emails are to be processed.
Put simply, this is not how to use the tool.
Pre-receive hooks that verify the Git commit message, the modified files, and implement similar code change controls used to be requirements of large enterprises working in regulated industries only....
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