Bettina, you have configured the 'Mail Server' resource in the QMC for the server-side, and in the Desktop client you did the same in the Settings\User Preferences\Mail tab?
The tricks after that are ensuring the condition is met to trigger things along with being sure you choose interactive option for Desktop and Batch for server-side.
About the best I have with the information provided, please leave further details if need further help. I am assuming this is in relation to Alerts option in the Desktop Client as well, correct if that is a bad assumption. Screenshots would be helpful in this case...
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