eml is the Outlook Express mail-format. While there are other programs that can read it, it's not universal, as you experienced.

The issue is comparable to what you see if you send, say, a Quickbook database to somebody who doens't have a Quickbook compatible program: he can't open it.

So the simple remedy: don't send it as an attachment. Open that mail in your copy of Outlook Express and forward it. Now everybody can read it.