From the ZepOps Blog:
Q: What does the vulnerability allow:
A: The vulnerability allows to run remote code in the context of MobileMail (iOS 12) or maild (iOS 13). Successful exploitation of this vulnerability would allow the attacker to leak, modify, and delete emails. Additional kernel vulnerability would provide full device access – we suspect that these attackers had another vulnerability. It is currently under investigation.
So, unless the attacker has another exploit they can take advantage of, the only thing they can do with this exploit is, "leak, modify or delete" emails.
Anyway, the fix is already in the beta so it should be out soon.