After experimenting a lot with several ways, I came to the following specification of securing the mails in the cloud that combines easiness and high speed:
Preparation: have the complete mail folder in an encrypted file container and copy it in the cloud.
- synchronise the remote container to the local one (copy only the differences)
- mount the local container on a directory
- fetch the mail from the mail server
- edit the mailbox with a MUA
- unmount the container
- synchronise the local container to the remote one (copy only the differences)
A back up of the local container is recommended.
The process is very easy and can be automated with a shell script. In this way the mails are safe in the cloud as well as locally.