我注意到安装 sendmail，然后卸载它，使用 apt-get 会留下许多不需要的文件。
sudo apt-get install sendmail
Reading package lists... Done
Building dependency tree
Reading state information... Done
The following extra packages will be installed:
liblockfile-bin liblockfile1 make procmail sendmail-base sendmail-bin
sendmail-cf sensible-mda
Suggested packages:
make-doc sendmail-doc rmail logcheck resolvconf sasl2-bin
Recommended packages:
default-mta mail-transport-agent fetchmail
The following NEW packages will be installed:
liblockfile-bin liblockfile1 make procmail sendmail sendmail-base
sendmail-bin sendmail-cf sensible-mda
0 upgraded, 9 newly installed, 0 to remove and 0 not upgraded.
Need to get 2256 kB …