I have Googled this error message and this appears to be an issue with the Windows Mobile operating system, and not specific to the ES400. I found the following on a post on this subject (link to the post follows):
Yes.  I figured it out.  After weeks of downloading multiple 3rd party email applications I also created a windows live email address which can receive emails from my work email address and push them to my device and I could open them in the default messaging program.  

I found that I could open the .eml attachments using the default messenger through my microsoft live account which was strange and all I ever wanted but when I replied, I did not want it to be from my live account, I wanted it from my work address.  

I went through all of the settings to see what could possibly be different and found the live account was set up as a pop3 and my work account was imap4.  

I deleted my imap4 settings for my work mail and created a pop3 for the same address and now I can open .eml & .msg email attachments.  Yay! 

I don't know why it works on POP but not IMAP, but it does.  Let me know if this helps. http://social.technet.microsoft.com/Forums/en/exchangesvrmobility/threa…