Why iPad can't open an email with 'open in mail' option? I have a stored email (.eml) file that can be downloaded and open with Mail.app on mac.
When I tried to open the same email in iPad, I clicked 'open with mail' to launch mail app, but nothing seems to happen; the mail app doesn't show the downloaded email. 

What might be wrong? Do I need to make additional setup for reading emails downloaded somewhere?
.eml is not supported by iOS by default.

You might try an external app like Klammer.