Temporary Lookup Failure = FQDN is not found in your network

your DNS server does not know the address

Solution: you mostly mistyped the mail servers address, that you did not share with us :)

Thank's for your quick response !

But before, the notifications were active, and overnight, I had nothing.
I didn’t touch the parameters..

It's weird no?

please try to ping your dns and your mailserver from the open project server

I've ping my dns and my mailserver from openproject ( smtp.host.docker.internal.fr ), and it's the same things..
I'm lost right now.. I don't understand..

Please share logs, terminal, screenshots, we could help you.

For reporting a request there is a general way of sharing your environment to us and also the open project version and the underlying OS

Waiting for more details

Hello, Sorry I'm late for the answer, You'll can see my setup on OpenProject

It's the same specs, 

Thanks

honestly, i do not see that you can ping your dns

i think you wrote smtp.host.docker.internal.fr and in the screenshot you removed the hostname and the top level domain and you just put host.docker.internal