Hey there, @andymac
The email you received is most likely to do with your Optusnet Webmail account. Your mailbox has 500MB of storage space so if it's getting close to being full, you'll receive notifications from us letting you know about this. You'll need to start deleting some emails or else once it's full, you won't be able to receive emails anymore
We have a Support Page that explains this in further detail.
To free up space in your inbox, start deleting unwanted emails in your inbox, trash and spam folder. If you need help or instructions on how to delete your emails, you can refer to our Webmail Help Guide
Let us know how you go!