I can’'t find the related topic in the FAQ or KB and turn to you for help. 
I just purchase new hosting plan with DH. I would like to transfer all my old emails using Horde from my old hosting to DH. What should I do? I already backup (using FTP) the Inbox file. Don’t know what format they are in, since there is no file extension after the file name. I open it with my WordPad and turns out to be just text file. Do I need to convert them, so SquirrelMail able to recongize it? Thanks.
Ask your old host – it sounds to me like the old file is mbox format (though I’d check to make sure!)… in which case you’ll need to convert this to Maildir format for Dreamhost.
This is a project I need to tackle as well somewhere along the line… I’ll write back here if I get to it soon.
Found the solution for this by backing up my email through IMAP. I use POP3 email access most of the time, but IMAP is also available. I use Symphony IMAP Backup software to backup all my email. It’s free and easy.
For those of you are beginners like me, I have found a perfect solution to this issue. IMAPSize!! Tested and works beautifully. It’s a free software with powerful features.
http://www.broobles.com/imapsize/
Please ignore the previous message with Symphony IMAP Backup; which caused too much crashes.
Great recommendation - exactly what I needed. Too bad DreamHost doesn’t give you access to backup IMAP mail by regular FTP 
mxxxxx dirs - I can ‘cd’ to the home dir of my mxxxx dirs, but not into the maildirs. I suppose the mxxxxx dirs are not really “owned” by my login, but by the mail user who may not want me to access them.
I guess the answer is to create regular ftp/telnet users if you want to backup by FTP.