Há uma pergunta semelhante em Microsoft (Mas não está claro se é sobre lembretes de correio)
Citando a resposta:
Run the Command given below and check if it helps:
Click on Start< All Programs< Accessories< Run< type ‘Outlook /cleanreminder’ (without quotes)
Note: Ensure there is a space between ‘Outlook’ and ‘/’
Also repair the PST file and check if it helps:
http://office.microsoft.com/en-us/outlook-help/repair-outlook-data-files-pst-and-ost-HA010354964.aspx?CTT=1
If the above steps do not fix the issue create a new profile and check if it works fine:
http://support.microsoft.com/kb/829918