Hillary Clinton's use of a private email server while she was Secretary of State has been one of the hot topics in the US presidential election, with Donald Trump threatening to prosecute (and jail her).

Now, in an interesting twist of irony, it's been revealed that a number of email servers linked to Donald Trump's hotel and other businesses are using outdated software with no security patches.

Security architect Kevin Beaumont, who uncovered the issue with Trump's servers, said a number of email servers for TrumpOrg.com, a domain registered to The Trump Organization, are still running Windows Server 2003 and IIS 6.0 — all out of date and unpatched:

Quick update on Trump corp email servers - all internet accessible, single factor auth, no MDM, Win2003, no security patching. pic.twitter.com/nIMTa9UmdL — Kevin Beaumont (@GossiTheDog) October 17, 2016

The Outlook Web App is here: https://t.co/VG723VcOAh - everything from OS to IIS to Exchange is unsupported. ActiveSync running too. — Kevin Beaumont (@GossiTheDog) October 17, 2016

The OS, by the way, is the one Clinton's private email server run and she got in trouble for. — Kevin Beaumont (@GossiTheDog) October 17, 2016

"Not sure what to say re Trump's email server. Other than during a hot topic of election campaign, maybe run an supported, patchable system," Beaumont tweeted.

"Windows Server 2003, IIS 6 and Exchange 2003 went end of life years ago. There are no security fixes. They don't have basics down."

Beaumont clarified that he's looking at public records and information available to everyone.

For instance, if you click on webmail.trumporg.com and open Ctrl+Shift+I in Chrome, the website advertises as II6 (Windows 2003).

Image: Screengrab

Microsoft says on its official website that it "will no longer issue security updates for any version of Windows Server 2003".

"If you are still running Windows Server 2003 in your data center, you need to take steps now to plan and execute a migration strategy to protect your infrastructure," it adds.

Republicans are up in arms again over Clinton's email because of an FBI document that appeared to show a senior state department official attempting to haggle with an FBI employee to downgrade an email marked as classified.

The exchange came to light as part of the fourth installment of Clinton email dumps from the FBI, this one containing 100 pages. (The FBI completed its investigation into Clinton's emails in July and recommended no charges.)