Why notification toggle on sidebar for issues sometimes can't be enabled

I don’t understand why I can subscribe to some issues and can’t to others using Notification toggle on sidebar.

For example, I can enable Notification for this issue:

And I can’t enable Notification for this issue:

Hi @AndreySV and welcome to the GitLab Community forum! :tada:

If you can’t enable notifications, that means the project owner has disabled email notifications be checkiing the “Disable email notifications” in the Project settings.


Thank you for the answer. I’ve asked project’s owner about this settings (Mobian · GitLab) and he told me that “Disable email notifications” option is not set.
I’ve created issue in my own project (Disable email notifications is not set):
test issue (#1) · Issues · Andrey Skvortsov / mutt · GitLab

and tried to enable notification for another created test account. Under test account I can’t enable Notifications for the issue (subscribe to updates). Slider is blocked.

This options is disabled like on your screenshot. Are there any other options that could cause this?