When you backup and store your email messages to disk or CD / DVD there are some requirements to keep in mind:
1. You should store the messages as single individual files: Storing the email messages as single individual files to disk as opposed to saving them in single monolithic files has some important advantages:
- It's easy to perform an incremental backup because you only need to copy the new messages instead of copying a big file.
- Because of this, is easy and fast to synchronize the messages to a new media or destination like Dropbox
- File corruption is most likely to happen in a big monolithic file.
2. You should store the messages in a format that can be read by only clicking on them. The mime format is a standard and is a good choice but if your system doesn't have an email client installed, it will not be easily readable. For best legibility you could use PDF or MHT. Probably PDF would be the best choice for long term storage as you can embed the email attachments and graphics within and because it is a universal adopted document storage format and is widely available.
3. The file names and folder structure used should allow you to search and find a particular message by using only your operating system file explorer.
4. When storing your messages offline you probably want to keep them private and should encrypt them. A very good format is the ZIP format because it has AES encryption and is a standard. Every email message should be stored in a single ZIP file. If you are going to encrypt your messages just make sure that you remember the password used.
5. You should store your email messages offline to an external disk, CD / DVD or to an online service because hard disks can fail. Storing your messages to Amazon S3 or a service like Dropbox or Google Drive would also be highly recommended because of its availability and security.