Hosted with DreamHost on their email servers. All data is encrypted as it enters their infrastructure. They control this encryption. This means that a request for data via a court order will render your email as readable, as DreamHost complies with authorities and court orders.
PLEASE NOTE THAT EMAIL IS NOT A PRIVATE MEANS OF COMMUNICATION
Storage of email is backed up and redundant as long as DreamHost is hosting their email server. Please contact the system admin if more mailbox storage is needed. Likewise, consider sensitive data to be sensitive and retain your own back ups. This might include an email archive that is stored on Nextcloud.
Nextcloud
All content within Nextcloud is stored on the local server and effects your storage quota. Which can be found in the bottom left corner of the window when logged in. If more storage is needed please contact the system admin.
The server itself is backed up nightly to a remote cloud storage that is based in the EU. The backup process runs every night as an incremental backup (changes only) and a monthly full backup. Both are encrypted locally, uploaded over an SSL connection, then encrypted again when received by the cloud service. The cloud service does have the ability to decrypt the data that it receives if requested by authorities. However, the first layer of encryption will still be intact and disallow the viewing of content.
Please be aware that this is not insurance for unsavory content being hidden from the view of authorities. The aim in privacy is to stop, or at the very least, restrict targeting advertisement.