Notes about the MAILHOST file extension:
To open this file type, a required software must be installed. Without it you only see a Windows message "The following file cannot be opened
" (Windows 7) or "How do you want to open this file?
" (Windows 10) if you double-click the file. There click on "More apps"
and choose a suitable software to view such files on your PC or from email attachments.
The MAILHOST file type is primarily associated with 'MSN Explorer' by Microsoft Corporation. If you are an MSN subscriber and having trouble with their service please contact MSN support for help. If you have a specific problem with the .MAILHOST file extension you can try this solution which has been confirmed by MSN to work: Click on the Start and then Run items. In the dialog box type:regsvr32 c:\progra~1\msn\msncorefiles\mailui.dll
...and then click on OK or press the Enter key (use cut/paste to be certain you've got it right). In case it wraps in your display, this is all one line with a space between the regsvr32 and pathname. This reregisters the mailui.dll file and solves the problem. If you are signed into MSN, sign out and then sign back in and everything should work OK. If, for some reason it does not, please go back to step one and contact MSN support.
Detailed information for file extension MAILHOST:
Program ID: MSMail.MessageList.View
To change file associations press Windows Key
and then Apps and Features
> Default apps
> Choose default apps by file type
Invalid file associations could be the result of underlying issues within your Windows PC.