A customer of mine, who runs Microsoft Office Outlook 2007, installed the Mozilla Thunderbird email application on his workstation. Somewhere along the way, either during the installation or when my customer first opened the Thunderbird client, Thunderbird was configured as the default mail client. Mail clients often check at startup and provide the user the option to set the mail client that’s being opened as the default, as shown in Figure 1.

All client OSs have default email client functionality. This is the email client that other applications call when they want to send something by email. Examples of this are the Send To: command in some Windows applications, including Microsoft Office, or the MailTo: command in a web application. When you click a link to contact a company through a website, it may use the MailTo: command to call the local default email client on the workstation. This is a protocol standard as outlined in RFC 2368, whether you’re running Vista or Ubuntu, XP or a Mac.

Whenever the customer who installed Thunderbird used the Send To: functionality in Word, Thunderbird opened when he really wanted Outlook 2007. Reversing the call to Thunderbird is an easy change either within Outlook or in the registry.

In Outlook, open Tools, Options and go to the Other tab. Check the box beside Make Outlook the default program for E-mail, Contacts, and Calendar, as shown in Figure 2, and click OK. Now when you open another email application, such as Mozilla Thunderbird or Windows Mail (formerly Outlook Express), make sure to not change the default and even check the box saying “Do not ask me again.”

Within the registry, the key to change the default mail client is stored in the HKEY_CLASSES_ROOT hive as follows:

\[HKEY_CLASSES_ROOT\mailto\shell\open\command\] @="\"C:\\PROGRAM FILES\\MICROSOFT\\Office<version>\\OUTLOOK.EXE\" -c IPM.Note /m \"%1\""

Where Office <version> determines the release of Outlook as follows:

Office 10 - Outlook 2002

Office 11 - Outlook 2003

Office 12 - Outlook 2007

This tip really has not changed over the years and through many versions, but it might be good to know it is still relevant with the most current releases of Windows and Outlook.