Customers don't receive all of the mail notifications

Hello Atlassian Community!
I’m experiencing an issue with mail notifications sent to customers.

The only notification our customers receive is when they create a new ticket by sending an email.

After that, no further notifications are sent. I’ve tried adding watchers, changing statuses, adding comments, and mentioning the customer with @, but none of these actions trigger a mail notification.

The user can still send an email with the correct issue key, and it will be added as a comment.

Most of our customers are within our organization (same domain), but we also work with some external vendors. I’ve tested this issue primarily with a test account created on @outlook.com (only using emails, without logging into an Atlassian account or Help Center), and the same problem occurred for tickets created by customers within our domain.

I’ve followed all the steps in How to troubleshoot customer notifications in Jira Service Management Cloud | Jira | Atlassian Documentation, but they didn’t help:

All notifications are enabled.

No translations are missing.

Customers are added to the projects and to the jira-servicemanagement-customers group.

There’s nothing in the Customer Notification logs.

I also noticed that the test account doesn’t display its email address in the project (there’s no Atlassian account associated with this email address).

Any advice or insights would be greatly appreciated!