You should be aware that access to your mail in Maildir format is only possible if you set up a full “user” on your domain for each mail account you want to backup. Then when you create a new mail address you have to select the option that says “also deliver to this DreamHost mailbox:” and in the dropdown that says “Mailbox Login:” select the user you just created.
You will then be able to login either via FTP or shell (depending on which you selected) and access the files to backup. If you created your user with shell access then yes you can use rsync to backup the maildir files. If not you can use an FTP client to do it.
If you don’t create a full domain user as above then you’ll get a mXXXX type mail user that you cannot access the files of. If that is the case your only alternatives are a) hope that DreamHost support can recover your mail account in the event of data loss or b) use one of the many IMAP backup programs that will let your backup ANY mail account regardless of whether you can access its maildir files.
I’ve read messages from people who did get DreamHost support to recover their email for them - I’m not sure what the circumstances were but regardless I would recommend you do your own backups.
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