.eml files are used by Outlook, so yes, if MSIMM.exe is associated with the .eml extension, that is the correct application.
Back in June, Microsoft posted a fix in it's knowledge base for Outlook users that couldn't open the .eml file. The link is:
http://support.microsoft.com/default.aspx?scid=kb;en-us;312355
CAUSE
This behavior can occur if an installed third-party e-mail program has an associated .eml file type.
RESOLUTION
To resolve this behavior, either run the Msimn executable file with the /reg option to force Outlook Express to re-register its file types, or re-register the .eml file type in My Computer.
To run the Msimn executable file with the /reg option:
Quit Outlook Express.
Click Start, and then click Run.
In the Open box, type msimn /reg, and then click OK.
To re-register the .eml file type in My Computer:
On the desktop, double-click My Computer.
On the Tools menu, click Folder Options.
Click the File Types tab.
Under Registered file types, click EML Outlook Express Mail Message, and then click Advanced.
Click Open, and then click Edit.
Type the following single line in the Application used to perform action box:
"C:\Program Files\Outlook Express\MSIMN.EXE" /eml:%1
Click OK, click OK again, and then click Close.