What does `mailto:` do when there is no email client?

Question

I am developing a website.

What does mailto: open in if there is no email client (like Outlook, Thunderbird, etc.)? It works on my computer, which has Outlo

Answer 1

What happens is entirely up to the client. The OS defines protocol handlers for protocols like mailto: or tel:, etc.

You would need access to the client's registry (in case of a Windows system) to manipulate the handling application for your protocol handler.

For Outlook 2013 as the designated handler, the according Registry structure looks like this:

[HKEY_CLASSES_ROOT\mailto]
@="URL:mailto"
"EditFlags"=hex:02,00,00,00
"URL Protocol"=""

[HKEY_CLASSES_ROOT\mailto\DefaultIcon]
@="C:\\PROGRA~2\\MICROS~1\\Office15\\OUTLOOK.EXE,-9403"

[HKEY_CLASSES_ROOT\mailto\shell]
@="open"

[HKEY_CLASSES_ROOT\mailto\shell\open]

[HKEY_CLASSES_ROOT\mailto\shell\open\command]
@="\"C:\\PROGRA~2\\MICROS~1\\Office15\\OUTLOOK.EXE\" -c IPM.Note /mailto \"%1\""

with a corresponding structure under HKCU.