Hi @Dietrich, Nathalie ,
How to enable when the Out of Office Assistant is enabled, that always a reply is sent to each sender, when you receive multiple messages from that person.
It's by design that when Out of Office Assistant is enabled, only one reply is sent to each sender even if you receive multiple messages from that person. Here's an official document about this: Only one reply is sent to each sender when the Out of Office Assistant is enabled.
Or do you mean you want to have a reply sent for every message? If this is the case, as mentioned in the last sentence of the linked article above, use a rule instead. Regarding your concern about the rule requires Outlook left running, instead of selecting "reply using a specific template." as the rule action, choose "have server reply using a specific message" so that it can work while Outlook is not running (Because it's a server rule).
As shown in the image below, when creating the rule, select "have server reply using a specific message", hit "a specific message", fill out the subject and email body, click the Save button at the upper left corner, close the message window and then follow the rules wizard to complete the setup.
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