Postfix : Port 25 is not opened in Linux, opened in firewall

Port 25 is not in opened port list

If you have installed postfix mail server and port 25 is not visible in opened port list. 
Have you opened it in iptables ( firewall ) ?
The line should be included in iptables.

-A INPUT -p tcp -m state -m tcp --dport 25 --state NEW -j ACCEPT

Yes, Yes, it is opened in firewall. I do not want this solution. 
Okay Okay

Open configuration file of postfix. /etc/postfix/main.cf
Check the parameter inet_interface, it should be

inet_interface=all

Now restart postfix again. It will be listed in opened port list.

lvm device not found in /dev/mapper

lvm device is missing. lvm Volume status is not available. Missing Volume.

If you are not able to find your logical volume (lvm device), check the status of the device or volume.
Run command

lvdisplay

If LV status is Not Available, it will not be listed in /dev/mapper or your lvm device directory.
Make the status available, Run command

vgchange -a y

It makes status available of all volumes.

Now the device will be visible in /dev/mapper or your lvm device directory.
Mount the device using

mount -t ext4 /path/of/the/lvm/device /path/of/mounted/location

Suppose I want to mount vg-lv_home in /home, I run

mount -t ext4 /dev/mapper/vg-lv_home /home

Now I can see all my files of the volume in /home

lvm commands not found

-bash: pvscan: command not found
-bash: lvdisplay: command not found
-bash: vgcreate: command not found
-bash: lvcreate: command not found
-bash: lvscan: command not found
-bash: pvdisplay: command not found
-bash: lvm: command not found

If you are using CentOS or fedora or RHEL and you are getting these errors again and again when you try to run lvm commands. It means lvm is not installed or corrupted on your system. Reinstall lvm again.

yum install lvm2

If it is not able to find lvm2 package and shows

No package lvm2 available.

It means your repositories do not have this package.
Enable appropriate repository like base, remi, rpmforge or epel and install it again.

If you have enabled all available repositories and still it shows "No package available."
Check the file /etc/yum.conf
The lvm2 package should not be there in exclude packages.
Repos do not list exclude packages and do not install them. If it is under exclude attribute, remove it and run install command again.
It should be installed.