The mailto one is invalid. There's nothing wrong or unexpected about URLs having different URI schemes than http or https. In fact you can configure the browser to send mailto: links to your email website instead of launching an application.
There's nothing wrong with linking different schemes as long as the user is expecting it. Most users click links without first examining the URL and the default assumption is that a link points to another page. If you say "click here to email me" or if the link text is an email address or whatever then, yeah, they'll expect a mailto. But if you don't get specific indicators like that, the user will tend to be surprised when their email is suddenly opening, especially if this involves opening Outlook or such, and it's not a good experience to unexpectedly be looking at the "compose new email" window.
Comments
The mailto one is invalid. There's nothing wrong or unexpected about URLs having different URI schemes than http or https. In fact you can configure the browser to send mailto: links to your email website instead of launching an application.
There's nothing wrong with linking different schemes as long as the user is expecting it. Most users click links without first examining the URL and the default assumption is that a link points to another page. If you say "click here to email me" or if the link text is an email address or whatever then, yeah, they'll expect a mailto. But if you don't get specific indicators like that, the user will tend to be surprised when their email is suddenly opening, especially if this involves opening Outlook or such, and it's not a good experience to unexpectedly be looking at the "compose new email" window.