GroupOffice should use it's own or an alternate IMAP library.
The problem with depending on the c-client library particularly kicks in when using SSL only mailservers. The PHP host and also the c-client library itself have to be (re)built with ssl support. This is an unnescessary dependancy.
The Iloha IMAP Library (IIL) is a good GPL example of an alternative IMAP library that doesn't rely on the standard C-Client based version. This allows the mail to function regardless of whether or not the PHP build it's running on has IMAP functionality built-in.
The IIL library is easy to implement and widely used (for example by RoundCube)