В вашем случае вы можете использовать android.util.Patterns package
.
EditText email = (EditText)findViewById(R.id.user_email);
if(Patterns.EMAIL_ADDRESS.matcher(email.getText().toString()).matches())
Toast.makeText(this, "Email is VALID.", Toast.LENGTH_SHORT).show();
else
Toast.makeText(this, "Email is INVALID.", Toast.LENGTH_SHORT).show();