Empty line in subscriptions file crashes Outlook over IMAP
ISPConfig 3, Debian 5.0.7, Linux 2.6.26 x86_64, "Perfect Server" Guide 
Accessing a virtual mailbox via IMAP with Outlook 2010 leads to reproducible crashes of Outlook.
Removing the empty first line from the files subscriptions and courierimapsubscribed fixes it.
There are least three error reports in a thread at HowtoForge .