MoboMix only supports sending plain text, as certain characters are rendered differently on different mobile telephones. Any character copied from an outside source can appear incorrectly. If you are copying and pasting from an outside source (such as a Word file), retype it manually, because there can be hidden characters. You can test this by sending the message to your own phone.
Also, handsets render characters differently, so you can try changing your phone settings.
We allow the following characters: a-z, A-Z, 0-9 and these special characters: .,:;!?()~=+-_\/@$#&%