Mentions in comments don't work

[EnriqSSSS]

Nextcloud version: 28.0.4(it also happened in a 29.0.4)
Nextcloud-onlyoffice integration app version: 9.3.0
Onlyoffice document server version: I don't know, I don't have access to it, and I don't know how to check, but probably latest
Browser: the same problem happened on both latest versions of chrome and Firefox. It doesn't seem to be a problem with the browser

Issue:
When mentioning in a comment ("+" followed by a mail address) doesn't work properly. When "+" is typed doesn't appear the list of contacts, and after typing a mail address the mail address turns blue(wich is expected) but no email notification is send.
Tested with a shared and a non-shared file, it didn't work in any situation.

Similar issues:
#897
#533

[EnriqSSSS]

Also related: #421

[SergeyKorneyev]

Hello,
The implementation of the Mentions feature in the Nextcloud integration currently has the following limitations:

1. You can only mention users that have an email address specified in their profile
2. The user you mention will receive a notification in the Notifications menu in the top right corner of Nextcloud's interface, but they will not receive an email, so the scenario you describe is working as intended. We'll check if it's possible to properly set up sending notifications via email and consider implementing this in the future