sendmail looks to have been compiled using the old file locking emulation from the old pre-2.2.x libraries. 2.2.x (2.1.x+) have completely gotten rid of the old file locking emulation and now use real BSD file locking. Because of this, sendmail (and any other program compiled with the old routines) will not be able to get any locks on an NFS share. Please recompile sendmail with the new libraries. rpm and elm are two other examples of this same problem.
Sendmail has been recompiled, but I suspect the problem lies elsewhere (knfsd?). I coalescing this problem with elm *** This bug has been marked as a duplicate of 4272 ***