I've copied the "home" directory of my old postfix mail server into my directory on the Zimbra server, and written a little script that parses it and then generates a list of zmmailbox commands like:

createFolder /old_mail
createFolder "/old_mail/Policy"
addMessage "/old_mail/Policy" "/home/jendreka/home/jendreka/Maildir/.Policy/cur"
addMessage "/old_mail/Policy" "/home/jendreka/home/jendreka/Maildir/.Policy/new"
createFolder "/old_mail/Sent"
addMessage "/old_mail/Sent" "/home/jendreka/home/jendreka/Maildir/.Sent/cur"
addMessage "/old_mail/Sent" "/home/jendreka/home/jendreka/Maildir/.Sent/new"
And then I run the import with:
/opt/zimbra/bin/zmmailbox -z -f /home/jendreka/mail_import_commands.txt

Everything seems to work great up until it throws the error:

ERROR: service.FAILURE (system failure: ZimbraLdapContext)
And then the whole thing falls over. The list of commands I'm feeding it is about 45,000 lines long, and it doesn't seem to die at the same place twice, so I don't think it's related to a specific user/email. I wrote another script that breaks the chunks of commands up by user, and waits 3 seconds between users, and it goes for longer, but ultimately it still dies. I get the feeling I'm overwhelming something. Any direction would be greatly appreciated.