Sending html mail displays correctly on some applications but not on others?

Sending html mail displays correctly on some applications but not on others?

I am sending emails via a C# application. The following characters
displays correctly on some email applications but on others, I get a weird
replacement (See attached image). How can I go about making these
characters always display correctly?
The character set : ( ) & * % $ # @ ! ~ ; _ = + / - ?
The Message : 18cm – 22cm Wide; Fat O
The result :
No matter which one of the characters gets used, The result is always the
same as the attached image.