As a default, you have 1GB of disk space at your disposal. You can increase this to 4GB with a paid upgrade.
An email alert will be send to you once you reach the limit of 90% of your storage space.
Once you reach 99%, all incoming emails will be rejected. This means that you won't be able to receive any emails until you have liberated some space by downloading or deleting old emails.
Connect to https://webmail.interact.lu/, enter your email address and password and click on "Connexion".
In the lower left egde of the inbox screen, there is a small icon that displays the current disk space usage.