If you need to work around international language support bugs while they are being worked on, or if you must for some reason use the least common denominator of all email systems, you can use the characters of the so-called ISO 646 subset.
The ISO 646 subset is roughly a subset of good old ASCII which has shown to be relatively safe to misinterpretation and corruption in tests.
Its characters, the characters absolutely safe to use in plain text email, are the following:
- a b c d e f g h i j k l m n o p q r s t u v w x y z
- A B C D E F G H I J K L M N O P Q R S T U V W X Y Z
- 0 1 2 3 4 5 6 7 8 9
- " % & ' ( ) * + , - . / : ; < = > ? _