Well, that’s a problem. Your regex will match the actual From header of emails. Are you sure that your separator between emails contains a colon? Usually it looks like From foo@example.com Sat Aug 18 19:23:56 2018. Note that there is no colon after the “From”, because that should be reserved for the email header.
If there is a colon in your separator, you might need to find another way to cleanly separate the emails in the mbox file. You are importing from mbox, right? Not just a bunch of emails where every email is stored in its own file?