Try not to conflate the two issues at play here:
1) It is NOT against the law to use external email for official purposes. It's is against federal guidelines but those don't carry the same weight as an actual law and usually only involve workplace sanctions, not legal ones.
2) It is against the law to store classified information on external servers.
3) It is against the law to hid or destroy federal documents which all SoS email are. Violation of this law carries a 3 year sentence and is one of the only laws I've ever heard of that also specifically states that violators can never hold public office in the US ever again.
Her defense for #2 is she claims that as SoS she never handled any classified emails ever, an almost impossible tasked but at least its a claim.
She is most certainly in violation of the retention laws for #3 however but depending on whether they can show she is only in violation of the hiding/withholding portion vs the destruction portion will most likely determine if anyone bothers to actually charge her or not.
They already have emails from other sources showing her having email discussions about semi-work related intel gathering with non-state employees. They may skirt the definition of official record enough for her to get away with not having retained those but if they find anything else on some third party server (say any foreign diplomat she exchanged emails with) but cannot find a corresponding copy on her server then she's done for.