j'ai trouvé ça sur le net (ayant eu le même souci, et cette solution a corrigé le problème - sans l'expliquer)
The problem you are experiencing most often occurs when the Outlook profile is corrupt. I recommend you use following steps to create a new profile.
Create a new Outlook profile
1. Close Outlook.
2. Click Start menu-> Control Panel. If Control Panel is not in Classic
View, click Switch to Classic View, and then double-click the Mail icon.
3. Click Show Profiles and then click Add. In the New Profile dialog box,
type a descriptive name for the new profile in the Profile Name box, and then
4. In the E-mail Accounts dialog box, click "Add a new e-mail account", and
then click Next.
5. Select the appropriate server type for your e-mail account, and then
6. Fill in the appropriate information as prompted, and then click Next and
Finish as needed.
7. Click the "Prompt for a profile to be used" option and click OK. This
way, you will be prompted for a profile when starting Outlook.
8. Restart Outlook and select the new profile.
Test if the problem is resolved. If the problem does not exist in the new profile, you can refer to the import and export section in following Microsoft Knowledge Base article to move your
data in the old profile to the new profile. Note: This article also applies to Outlook 2003.
Bien cordialement / bon courage.