Hi,
Thank you for your quick answer. Sorry but I'm not very good in Android to make a very precise answer.
NFC is an acronym for Near Field Communication, it's a contactless communication between 2 devices (distant of few centimeters max). In Android the NFC stack is deployed under Android Beam and allow to read NFC tags or exchange data between phones (vcard, picture email, bluetooth pairing...). Of course you need a smartphone with NFC capability and an NFC tag (or another smartphone with NFC) to play (or test) with it. (exemple on youtube, search "How to use NFC on Android").
The NDEF message is a format for the exchanged information between 2 NFC devices, this is automatically managed by Android beam. (This format is defined by the NFC Forum, and implemented in android)
I think that you only need to add
<intent-filter>
<action android:name="android.nfc.action.NDEF_DISCOVERED" />
<category android:name="android.intent.category.DEFAULT" />
<data android:scheme="mailto" />
</intent-filter>
I don't think you need to add permission to NFC as your application is not driving the NFC but as I said I'm not good enough in Android to be sure.
I tried to open a .eml file also (previously saved from thunderbird) but my phone listed only GMAIL and EMAIL. I don't know if there is a generic way to declare the application as a mail client for android.