Some notification mails are not sent

I’m running Gitlab on kubernetes inside a GKE cluster. It has been installed with helm chart v4.11.1.

I just update the smtp configuration and it’s working… Sometime. I mean I ran several pipeline today and I received notifications for some of them but some others did not.

I tried to find something in the logs but I could not find something useful.

My question is: how can I debug this ? Is their somewhere in the logs or in the user interface where I can the list of mails that has failed with the reason why it failed ?

Thank you.