Most normal characters can be sent out with TEXT encoding. However, when you want to use special characters like greek or chinese characters, you will need to use a UNICODE encoding. Be aware that UNICODE encoding means that you will only be able to write 70 characters per SMS.
A common mistake is when you try to send out an emoticon. When you type ";)" in the message area of SMS Mailer, it should go out without any problems. However, when you type ";)" in a text editor like MS Word, some editors will automatically replace that ";)" with an emoticon. When you then proceed to copy paste that text to the SMS Mailer, the emoticon will be copied as well. When that message goes out with TEXT encoding, the emoticon will be replace with "??".
For a complete overview of the allowed characters in TEXT encoding, please visit https://support.ringring.be/hc/en-us/articles/115005250203-What-characters-can-I-use-in-a-text-message-