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.
I run a quad core XP pro based machine. I use Outlook express and some friends can't open my attachments. They indicate attachments with an extension EML wont open. They are not using Outlook express. Anything I can do at my end to solve the problem?