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I'm usually using Thunderbird, but for testing purposes have to start Outlook every now and then. Afterwards, Thunderbird informs me that it is no longer the default client, which it should have remained.

How to stop Outlook 2010 from being so insolent?

In fact, Outlook does not change any of the settings I checked so far (it doesn't re-register files, and mailto: links still open Thunderbird, so I'm rather confused at what Thunderbird considers to be modified by Outlook

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Start -> Control Panel -> Default Programs.

Set your default programs.

Select your program from the list and then click on Set this program as default.

Or/And, Start -> Control Panel -> Default Programs.

Set program access and computer defaults


Choose default mail program.

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Confusingly, Outlook does not seem to change these, and mailto: still opens Thunderbird. I'm afraid I have to change my question into "What does Thunderbird consider modified by Outlook and how to stop this?" – Tobias Kienzler Nov 29 '12 at 8:59

In Outlook...

Go to Tools > Options. Then click on the Other tab. In here you will find a checkbox named "Make Outlook the default program for E-mail, Contacts, and Calendar". If this is unchecked then Outlook should not assume control each time it is loaded.

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In what program :) – Dave Nov 29 '12 at 9:34
Outlook. For 2010, select File->Options and you will find the option at the bottom of the first page (General) under Start up options. – Miles Lott Feb 8 at 16:26

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