In Red Hat Enterprise Linux ES 4.1 Update 3, both /etc/pam.d/imap and /etc/pam.d/pop contain hard-coded paths pointing to pam_stack.so, actually /lib/security/pam_stack.so. This causes problems on x86_64 since pam_stack.so is located under /lib64/security/pam_stack.so.
A workaround is to modify /etc/pam.d/imap and /etc/pam.d/pop to look like this:

#%PAM-1.0
auth required pam_stack.so [...]