MAILHOST file extension is used by Microsoft MSN Explorer to store user preferences for the email server. The file is in a Microsoft proprietary format and is required in order for MSN to work correctly. You cannot open MAILHOST file and modify the settings directly. The only way to change MAILHOST file is to modify settings from MSN application. The file will usually be named 'MSN#.mailhost', where '#' is the version of MSN. For example the file would be MSN9.mailhost on systems using MSN 9. If this file gets damaged or deleted, the MSN software will display an error message which can be rectified in the following way:
1. From the Start menu, choose Run...
2. Type 'regsvr32 c:\progra~1\msn\msncorefiles\mailui.dll'
This should fix the error and restore the MAILHOST file to a working state.
Another file extension related to this is DOWNLOADHOST, usually only found on the file MSN#.downloadhost. The DOWNLOADHOST file is also installed with MSN software and is partially responsible for checking email and tracking preferences related to downloading files through MSN. If you have a problem with this file the recommended cure is similar to the above and is performed as follows:
1. Select Run... from the Start menu.
2. Type 'regsvr32 c:\progra~1\msn\msncorefiles\msnmetal.dll'
This should restore the DOWNLOADHOST file and enable the MSN software to function properly once again.
These files cannot be opened directly or edited manually as they are simply settings files used by MSN software, the commands above are the only methods available at present for restoring these files to a working state.
There is also usually a file with the extension MSN used to store information on blocked internet hosts and websites. By default this file is located in the WINDOWS\system32\drivers\etc directory and is named hosts.msn. This file has been corrupted or damaged by several known malware programs to enforce restrictions on the internet usage of the target computer. In these cases generally rebooting into safe mode and performing a full system scan with anti-malware software will be able to fix it. Once the malware is removed the hosts.msn file can be deleted and replaced with the default hosts file.
For more general information about files, file extensions and registry you can read one of the following articles:
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