atmail only keeps limited backups in case of server issues. I would like to take my own backups for increased data integrity.
- atmail cloud
Below are some options that can be used for 3rd party backup solutions.
Email Forwarding Service
An email forwarding service is useful since it lets you easily change where you read your mail from, without getting a new address. You could use a email forwarding service like POBox that does not add any visible headers when it forwards a message to automatically store a copy of every new message in another IMAP account.
- Use imapsync to copy the messages. It requires that you install Perl. See this web page for a writeup with screen shots.
- Use MailSync . It is designed for a Unix/Linux like operating system though you may be able to run it under Windows if you also install cygwin. Its written in C so you don't need to install an interpreter such as Perl or Python.
- Use offlineimap . It is designed for a Unix/Linux like operating system though "With some fiddling, it reportedly ran under Windows as well".
- isync. Its also known as mbsync. Not to be confused with Apple's iSync program for mobile phones. It supports IMAP and Maildir mailboxes.
- Use a Windows utility like IMAPSize to backup the messages as .EML files. It does an incremental backup - it won't backup a message again if its already backed up. You can use it to restore any backed up messages or use the ImportExportTools add-on to import the *.EML files into Local Folders, and then move the messages to a remote folder. Since it supports command line arguments you could automatically run it using the Windows Task Scheduler. One advantage it has over a offline folder is that if you delete a message in a remote folder your local copy (a .EML file) is not deleted like it is with a offline folder.
- Use a Python script such as Imap Backup Tool to backup the folders as mbox, mbox.gz, or mbox.bz2 files. It supports incremental copying from any/all folders. It doesn't backup IMAP flags (tags). IMAPSave is another one. It only stores the messages in mbox files and does not do incremental backups. Both of these programs only do backup, but you could use the ImportExportTools add-on to import the mbox files.
- BeyondInbox sells for $9.99 a command line tool for Windows/Linux/OS X called Beyond Inbox Commander that does incremental backups of IMAP folders to .eml files. It also supports sending a backup report through mail after the backup is completed.
- imap2maildir is a Python script  to backup a IMAP mailbox to maildir files. Thunderbird 12 added support for maildir files but they're not useful yet as there is no support for converting mbox based accounts etc. yet.
- If you are using Linux or OS X you could use fetchmail and procmail to backup a IMAP mailbox to mbox or maildir files.
- IMAP Backup & Archiver will schedule backups of IMAP accounts to Amazon Web Services (EC2, S3).
There are lots of other programs/scripts to synchronize mailbox's or backup remote folders. However its best to stick with something well known that is actively maintained.