If an existing ticket number of the receiving Jira (Cloud) instance - i.e. the one where the mail handler is configured - is contained in the mail's subject, later emails are correctly treated as comments for the existing ticket.

If any such ID is part of a mail's subject, that does not exist as ticket in the receiving Jira instance (but f.ex. in another system or another Jira instance), this is not recognised and treated as the same issue but a new ticket is created by the mail handler for each single incoming email.

It would be highly beneficial if the mail handler offered additional configuration so that all emails with identical subject are treated as belonging to the same ticket.
The first email triggers the creation of a new ticket and all following email with the same subject create comments for this ticket.

This should not replace existion cinfugiration options of the mail handler but should be an additional option.

Examples for mail's subjects:
[External ID] [Text] or
[Text]