Dealing with Unicode

[Alex Schroeder]

Sometimes I'm writing an email that contains "Emoji" characters 
such as this one:

----------------------------------------------------------------------
            position: 219 of 800 (27%), column: 34
           character: 😍 (displayed as 😍) (codepoint 128525, 
           #o373015, 
#x1f60d)
   preferred charset: unicode (Unicode (ISO10646))
code point in charset: 0x1F60D
              syntax: w         which means: word
            category: .:Base
            to input: type "C-x 8 RET HEX-CODEPOINT" or "C-x 8 
            RET 
NAME"
         buffer code: #xF0 #x9F #x98 #x8D
           file code: #xF0 #x9F #x98 #x8D (encoded by coding 
           system 
utf-8)
             display: no font available

Character code properties: customize what to show
 name: SMILING FACE WITH HEART-SHAPED EYES
 general-category: So (Symbol, Other)
 decomposition: (128525) ('😍')
----------------------------------------------------------------------

After a few seconds, I get shown the buffer ` *Format Temp 0*' and 
a warning:

----------------------------------------------------------------------
These default coding systems were tried to encode text
in the buffer ` *Format Temp 0*':
 (emacs-mule (164 . 128525))
However, each of them encountered characters it couldn't encode:
 emacs-mule cannot encode these: 😍

Click on a character (or switch to this window by `C-x o'
and select the characters by RET) to jump to the place it appears,
where `C-u C-x =' will give information about it.

Select one of the safe coding systems listed below,
or cancel the writing with C-g and edit the buffer
  to remove or modify the problematic characters,
or specify any other coding system (and risk losing
  the problematic characters).

 utf-8 gb18030 utf-7 utf-16 utf-16be-with-signature
 utf-16le-with-signature utf-16be utf-16le utf-8-auto
 utf-8-with-signature utf-8-nfd utf-7-imap utf-8-emacs
----------------------------------------------------------------------

How can I prevent this error message? Why is Gnus (or Emacs?) 
using emacs-mule to save this buffer?

My .emacs contains this setting:

(prefer-coding-system 'utf-8)

Cheers
Alex