e-mail reply ending up as new e-mail

I'm using yahoo mail, and all my submissions to the Hobbit Group are interpreted as new e-mails. I thought the e-mail receiver would interpret e-mails with "RE: " in Subject as replies to existing e-mails. Alas, no.

How do I submit a reply to existing e-mails in Hobbit Group mail, as a Follow-Up?

Thank you

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