Hi Kng180_te,
I noticed that you have posted a English thread in our Chinese Tranditional Office 365 support forum. If you want to get English support, you’re welcome to post a new thread in our English forum via: http://community.office365.com/en-us/forums/default.aspx.
About the issue, according to the error message “550 5.6.0 CAT.InvalidContent.Exception: InvalidCharsetException, Character set name (x-gbk) is invalid or not installed”, the issue may occur if you use a un-supported character in the message.
According to this wiki page (http://technet.microsoft.com/en-us/library/aa998600.aspx), currently only the “big5” character is supported for Chinese Traditional . I suggest you change the settings in the message and try again.