123.reg mail

You're looking for a.reg file to configure your Windows registry!

A .reg file is a text file that contains registry settings in a format that can be imported into the Windows Registry. Here's a simple example of a .reg file that sets the default mail client to 123 Mail:

Windows Registry Editor Version 5.00

[HKEY_CLASSES_ROOT\mailto]
@="123 Mail"

[HKEY_CLASSES_ROOT\mailto\shell\open\command]
@="\"C:\\Program Files\\123 Mail\\123 Mail.exe\" \"%1\""

Let me explain what each line does:

1. Windows Registry Editor Version 5.00: This line specifies the version of the registry editor that created the file.
2. [HKEY_CLASSES_ROOT\mailto]: This line specifies the registry key that we're modifying. In this case, it's the mailto key under HKEY_CLASSES_ROOT, which is responsible for handling mailto links.
3. @="123 Mail": This line sets the value of the @ (default) key under mailto to "123 Mail". This sets the default name of the mail client.
4. [HKEY_CLASSES_ROOT\mailto\shell\open\command]: This line specifies the registry key that contains the command to launch the mail client.
5. @="\"C:\\Program Files\\123 Mail\\123 Mail.exe\" \"%1\"": This line sets the value of the @ (default) key under mailto\shell\open\command to the path of the 123 Mail executable, followed by the %1 parameter, which represents the URL or email address to open.

To use this .reg file, simply double-click on it and follow the prompts to import it into your registry. Note that you may need to adjust the path to the 123 Mail executable if it's installed in a different location on your system.