Before you do anything, take a moment to read this article on bug reporting, and keep its comments in mind when reporting Horde bugs.
Having read that, there are two places where a bug can be reported. If you're on the mailing list for the component you think you've found a bug in, send a message to the mailing list, and be sure to keep an eye out for replies. You can also enter the bug into the Horde bug database.
Be sure to describe exactly how to reproduce the bug (if you know!), and include all error messages that appeared. Also, please inform your local support staff of the bug; they will be able to provide us with information specific to your organization's configuration which may be necessary in resolving the problem.