"Mailto" link in Win 10 Pro not working on my system only.

basically, I agree with all that has been said, especially @Jon Fritz 's excellent answer.

However, I had a client who was adamant, he wanted a mailto link. According to him, people tend to use the link before filling in a form. So I gave him a mailto link.

I then decided to bring the proof to the pudding on my own website. Glory be. the guy is right. No one uses the form, they all use the mailto link.

It's irresponsible to post a MAILTO link on a website these days.  Spambots will exploit it which could cause your client's email address and URL to be blacklisted for spamming.  It's not worth the risks.

If the hosting server supports PHP, use a secure contact form with PHPMailer which hides email addresses from robot harvesters and prevents other exploits.

https://github.com/PHPMailer/PHPMailer

mailto:name@domain.com?subjet=Hello word&body=ready to go

I'll go a step further and say using a plain mailto: link isn't really acceptable in today's website design at all.

1. Security. As mentioned above, email harvesters eat up mailto links, it's what they're for. Your clients may not be very happy when their email address is used by a spammer resulting in a large corporate email filter blacklisting their entire domain.

2. Email client usage is down. Way, way down. Mailto links, as you've found, only function when the viewer has an email client installed on their device. For anyone who uses browser-based email, like Gmail through Chrome, the link will do absolutely nothing when clicked. 

3. Spotty support among email clients. Some will allow you to set the subject, body and other parameters, some won't. You can't control who uses which client, browser, etc. so you can't control how the email appears and what gets filled in.

4. Spam responses. Mailto: is open to any and all robots that love nothing more than to send your clients an email from a bogus Nigerian doctor, counterfeit little blue pill advertisements, and "Dear -client name here- I'm in jail, please send money" scams. With form-to-email scripts, you have the ability to validate human interactions and secure the form against robot intrusion, cutting out nearly all spam your client will get from a contact link.

Instead of using simple mailto: links, look into adding a form-to-email script in a language your client's servers support (usually PHP). You'll have a much more secure and consistent experience for your clients potential customers.

1. Some hosting companies offer simple, but secure, form scripts with easy instructions to implement them. Talk to the hosting company, or dig into their website to see if they have one available.

2. There are alaso a good number of well-made third party scripts available, though this is usually less ideal if you're not sure how to code forms and what security is necessary.

3. Finally, you can let a third party service, like Wufoo.com handle all the heavy lifting. They host the form and database, all you do is set up the desired form on their site and embed it in your client sites.

First, I wouldn't recommend ever posting emails publicly like this in contact links. They are just going to be harvested by bots and eventually those emails will get spammed. You are much better off setting up forms to capture and send the emails.

With that said, can we see the code you are using for your links? I have not come across that error before, but with mailto links there needs to be a configured mail client either online (ie: gmail) or as a desktop/phone app (ie: outlook).  Also as you are including a subject, have you encoded the subject so that there are no spaces and only encoded characters (ie: space characters become '%20')?