After adding more than 2000 users, only the first 500 get the activation email.
This is VIDIZMO's default behavior since VIDIZMO Exchange Server only allows emails to be sent to 500 recipients in one go.
VIDIZMO Exchange Server has limitation to send emails to max 500 Recipients. Either you can send emails to Max 500 Recipients each time or you can set up your own Exchange server with VIDIZMO.
The workaround to this is to send emails in batches of 500 or less. You can send the emails one after the other as there is no restriction on the duration between the first batch of users to the next batch of 500 users.
Click here on How To Setup Your Own SMTP Server for Email Notifications to learn more.