您好，我正在尝试使用 dovecot 和 postfix 配置邮件服务器，在安装 postfix 时出现以下错误，我不知道如何修复它，我搜索了几个小时却没有任何线索？
提前致谢！```
root@mail:~# apt install postfix -y
Reading package lists... Done
Building dependency tree
Reading state information... Done
Suggested packages:
procmail postfix-mysql postfix-pgsql postfix-ldap postfix-pcre sasl2-bin dovecot-common postfix-cdb mail-reader postfix-doc
The following NEW packages will be installed:
postfix
0 upgraded, 1 newly installed, 0 to remove and 0 not upgraded.
Need to get 1,152 kB of archives.
After this operation, 3,697 kB of additional disk space will be used.
Get:1 http://nyc2.mirrors.digitalocean.com/ubuntu xenial-updates/main amd64 postfix amd64 3.1.0-3ubuntu0.3 [1,152 kB]
Fetched 1,152 kB in 0s (1,875 kB/s)
Preconfiguring packages ...
Selecting previously unselected package postfix.
(Reading database ... 132715 files and directories currently installed.)
Preparing to unpack .../postfix_3.1.0-3ubuntu0.3_amd64.deb ...
Unpacking postfix (3.1.0-3ubuntu0.3) ...
Processing triggers for libc-bin (2.23-0ubuntu10) ...
Processing triggers for ufw (0.35-0ubuntu2) ...
Processing triggers for systemd (229-4ubuntu21.2) ...
Processing triggers for ureadahead (0.100.0-19) ...
Processing triggers for man-db (2.7.5-1) ...
Setting up postfix (3.1.0-3ubuntu0.3) ...
insserv: script postfix: service mail-transport-agent already provided!
insserv: exiting now!
update-rc.d: error: insserv rejected the script header
dpkg: error processing package postfix (--configure):
subprocess installed post-installation script returned error exit status 1
Processing triggers for libc-bin (2.23-0ubuntu10) ...
Processing triggers for systemd (229-4ubuntu21.2) ...
Processing triggers for ureadahead (0.100.0-19) ...
Processing triggers for ufw (0.35-0ubuntu2) ...
Errors were encountered while processing:
postfix
E: Sub-process /usr/bin/dpkg returned an error code (1)
```