You set your default encoding to UTF-8.
if you receive an email in an encoding other than
UTF-8, you cannot read it, although you have mb_string
for example, SM uses EUC as Japanese encoding, although
the std. might be set to UTF-8 (why?). So if you set
the langauge to Japanese, send an email to yourself and
then switch back to english, it cannot be read.
if utf-8 is used, the system should be automatically
converting the encoding with mb_string functions to
utf-8. Otherwise you can read only utf-8 sent-messages,
even though they come from the same copy of Squirrelmail.
Also I suggest _always_ using UTF-8 as encoding if the
default encoding if UTF-8. Why set it so if its not
done? UTF-8 was meant for that IMHO.