Bad dependency in the libsasl2-2 package
Looks like libsasl2-2 in backports has a bad dependency. It has a
conflict postfix (<< 2.3.4-3). Except postfix 2.1.5-9 in stable
doesn't depend on libsasl2 at all, so the dependency is bogus. If
anything, it should have a conflict against postfix-tls since that's
the version of postfix in sarge that depends on libsasl2.
The end result of the bad conflict is that you can't install the new
libsasl2 libraries and modules without aptitude trying to deinstall
the stable version of postfix.
Beer and Code Wrangler at Large