Acho que há algo errado com o seu DNS , pois não é possível localizar o host yum repo e dar o erro, tente ping google.com para ver se o DNS está funcionando ou não
yum remove postfix fez com que o próprio yum não funcione mais e nem o wget epel repository ... nada pode ser encontrado.
Nesse caso, é melhor reinstalar o Centos?
Could not retrieve mirrorlist http://mirrorlist.centos.org/? release=7&arch= error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch= error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch= error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch= error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
Could not retrieve mirrorlist http://mirrorlist.centos.org/?release=7&arch= error was
14: curl#6 - "Could not resolve host: mirrorlist.centos.org; Unknown error"
repo id repo name status
addons/7 CentOS-7 - Addons enabled: 0
base/7 CentOS-7 - Base enabled: 0
centosplus/7 CentOS-7 - Plus enabled: 0
contrib/7 CentOS-7 - Contrib disabled
extras/7 CentOS-7 - Extras enabled: 0
updates/7 CentOS-7 - Updates enabled: 0
repolist: 0
ID | Login user | Date and time | Action(s) | Altered
-------------------------------------------------------------------------------
85 | root <root> | 2015-11-29 15:20 | Erase | 2 EE
84 | root <root> | 2015-11-29 15:20 | Erase | 1
83 | root <root> | 2015-11-29 15:20 | Erase | 1 EE
82 | root <root> | 2015-11-29 11:28 | Erase | 1
81 | root <root> | 2015-11-29 11:22 | Install | 2
80 | root <root> | 2015-11-25 11:16 | Install | 1
79 | root <root> | 2015-11-25 11:16 | Erase | 1
78 | root <root> | 2015-11-25 10:32 | Install | 1
77 | root <root> | 2015-11-25 10:09 | Install | 1
76 | root <root> | 2015-11-25 09:48 | Install | 2
75 | root <root> | 2015-11-25 09:45 | I, U | 47 EE
74 | root <root> | 2015-11-24 18:58 | Erase | 1
73 | root <root> | 2015-11-24 18:58 | Erase | 1
72 | root <root> | 2015-11-24 17:18 | Install | 1
71 | root <root> | 2015-11-24 17:16 | Install | 1
70 | <me> | 2015-11-18 21:32 | Install | 1
69 | root <root> | 2015-11-18 21:27 | Erase | 1 EE
68 | root <root> | 2015-11-18 19:50 | I, U | 3
67 | root <root> | 2015-08-07 17:45 | Reinstall | 1
66 | root <root> | 2015-08-03 19:57 | Install | 127
Tags yum