As many readers know, I have my own domain which I use for email. What many may not know is that I am also a certified network technician, certified PC tech and I have operated my own email server for nearly 20 years. For 15 of those years I operated my server in my residence - just like Bill and Hillary Clinton. (In recent years I have leased a server in a data center, which I administer remotely.) All of this qualifies me to comment on the technical issues surrounding this controversy.
1) Security Issues
I am going to assume that the Clintons had enough money to hire a very good tech to make sure that they were not going to be hacked. Absent more information, IMHO this is not an issue.
2) Bogus Excuse
Today, Secretary Clinton stated that she used her personal email address at the Clinton's personal domain and server because she did not want to carry two devices. The reason this is a bogus excuse is that it is easy for most email clients to handle more than one email account. At one point I had four email accounts in one program: Personal, business, church and Civil Air Patrol. It's actually quite common. Hillary's excuse doesn't make sense.
3) Hidden and Easily Destroyed Communications
Someone who is using a governmental account for email creates an indestructible record. "Delete" does not destroy the data. As Col. Ollie North found out way back in the 1980s, the messages are recoverable not only from the original drive on the server that the account exists on, but on others it may have been relayed through, as well as on backup drives or tapes. Recently Lois Learner's emails - which we supposedly unrecoverable - were found on backup tapes. This is consistent with a failed effort to destroy them.
Someone using a private email system such as Gmail, Yahoo Mail or Outlook.com faces much the some challenge in getting rid of messages. Messages might be recoverable for a long time after the user deletes them.
When one moves to a private email server, everything changes. Someone running their own server has total control over everything. They can create and delete accounts. They can destroy backups. They can delete or even edit emails. Most importantly, after accounts and emails are deleted the drive can be scrubbed, making recovery impossible.
A government official - any government official - using their own server, could set up accounts not only for themselves, but for any other people they wished to communicate with - inside and outside government. In this case, emails between accounts on the same server would never show up anywhere outside of the private server and the computers or devices of the users. It would be a secure and completely private way for officials to communicate "off the radar". When the need for such communications ends, deleting the emails and running a secure delete or scrub program on the server and the devices would completely destroy any trace of the messages. The only evidence would be the absence of any deleted files from before the date the drive was scrubbed.
This is why the use of private email systems by government officials is so dangerous. It allows for completely secret communication that can later be completely destroyed. In short, exactly what laws passed after Watergate were passed to prevent. IMHO legislation should be passed post haste to require all governmental officials currently required to preserve communications to use government email accounts for ALL email communications - both private and work related.
It remains to be seen if Secretary Clinton used her private server to get around the law - but her deletion of mails she considered "personal" certainly raises suspicion. If she wants to establish that nothing illegal took place she can have an independent computer security firm examine the server. If it hasn't been scrubbed, then as many emails as possible should be recovered. An independent agency - such as the FBI can then determine if any of them are work related. If the drive has been scrubbed, the only reason to do that is to hide something..... either way, if she wants to be president we probably need to know.