Possibility to configure JIRA mail hanlder to mark emails as read (not deleted) after they are processed

Problem Definition

IMAP mailbox is queried for non “DELETED” messages, after processing messages are marked as “DELETED” and removed from the server.

Suggested Solution

It would be nice to have the possibility to configure mail handlers in JIRA to leave the messages on the mail server after they have been processed.

A similar behavior to the one seen in Service Desk when using IMAP:

If you use IMAP, emails are marked as read (not deleted) after they are processed by JIRA Service Desk.

(source: https://confluence.atlassian.com/servicedeskcloud/receiving-requests-by-email-747602718.html)

Note on fix

We will introduce a new "mark as seen" checkbox in the UI for handlers using IMAP.

If you decide to switch to "mark as seen" option, please make sure to delete emails from the IMAP folders periodically to prevent unlimited size growth.

Also:

• The current/old handlers should have it "off" by default so the behaviour should be the same as before: processing should delete email from IMAP.
• There is small change to the way the emails are fetched - before only "not deleted" has been fetched, now "not deleted" + "not seen" - this is to ensure that switching from "checked" to "unchecked" won't process the "seen" mail.
• The previous behaviour of fetch "not deleted" can be restored if needed by using feature flag "com.atlassian.jira.mailHandlerImapMessageQueryLegacy.enabled"