Up until recently the email description field or the body of the email have been stored within the Microsoft Dataverse data store. This is going to be changing in the coming months where the email description field will be moved to an Azure Blob Storage, which is also where the attachments, annotations, files and images are stored.
This new functionality has already started this last May (2023), and will continue for 6 to 12 weeks for existing customers (possibly longer based on the size of the data). After the initiate move of the data, and remaining migration will be a continuous process where all email descriptions that are over 12 months old will move to the Azure Blob storage automatically.
There are some limitations that are caused by this new move to Azure Blob storage as filtering and searching on the email description field will now be limited. Any email descriptions that are stored in Azure Blob storage (for emails that are older than 12 months), FetchXML queries, Quick Find and Advanced Find on the email body will not be supported. Dataverse search can be enabled on the email description in order to search for the email on email descriptions.