安装rpm包时报这个错是因为什么啊？

error: Failed dependencies:
dovecot = 1:1.2.0-1.fc13.nsmail.16 is needed by dovecot-devel-1:1.2.0-1.fc13.nsmail.16.i386

依赖阿，你需要安装dovecot这个包

[root@tangfu i386]# rpm -ivh dovecot-1.2.0-1.fc13.nsmail.16.i386.rpm
error: Failed dependencies:
lib20_mdec_plugin.so is needed by dovecot-1:1.2.0-1.fc13.nsmail.16.i386
lib20_menc_plugin.so is needed by dovecot-1:1.2.0-1.fc13.nsmail.16.i386

我现在装dovecot这个包怎么有出现这个错误啊？ lib20_mdec_plugin.so和lib20_menc_plugin.so这两个东西我在项目中找到了，可是它又不是rpm包， 该怎么安装啊？