Problem

If running on Mac OS X 10.7, Notes clients may hang at startup or upon launching or accessing the mail inbox.

Cause

The default semaphore mechanism for Notes conflicts with changes introduced in Mac OS X 10.7. See below for a possible workaround and information on future plans for support of Notes on Mac OS X 10.7.
Resolving the problem

Workaround for random hangs in pre-8.5.3 versions of Notes on Mac OS X 10.7

Edit 'Notes Preferences' and add the following line: POSIX_SEMAPHORES=1

NOTE:

This workaround is provided for informational purposes only and does not imply formal support for pre-8.5.3 versions of Notes on Mac OS X 10.7. Also, IBM does not have plans to formally test Mac OS X 10.7 with versions of Notes prior to 8.5.3, or to formally support this workaround for versions prior to Mac OS X 10.7.

Source: Random hangs at startup or when accessing mail in Notes on Mac OS X 10.7