BAK files use the normal file extension BAK.
Backup file is perhaps, the easiest to create in the system, and in the computer world.
You would come across plenty of applications where BAK extension is created on Windows.
Normally BAK files are created automatically, without you ever trying to create them.
The best example of BAK extension is seen when your operating system gets corrupted in the main file format.
The first step you take is perhaps, to go to the backup file location in the partition half of your hard disk, and get your backup Windows file to replace your corrupted one in the main format.
That is how BAK extension is used.
To open it is even easier.
Just click on the icon twice and your BAK application will open up.
There are occasions when your system may not support opening this format.
In such cases, you need to recheck your computer to find the necessary software required to run it.
The file extension MAILHOST is quite different from the BAK one.
MAILHOST file is only related to the MSN program.
It is an email checker in MSN mail box.
Talking of difference between MAILHOST and BAK extensions, the latter does not have a developer, while the former is developed by Microsoft.
BAK format is not related in any way to email or Internet applications, but MAILHOST is primarily an Internet-based application, which checks emails.
If your MAILHOST extension has become corrupt, then you need to go to the Windows start menu to rectify the error.