Each of your files (mailing list, text message, and HTML message) should be ANSI or ASCII encoded files. This means they should not be UTF-8 or Unicode. AutoMSW will give a warning upon starting a job if it detects UTF-8 or Unicode encoded files are being used.
When creating any of these files, make sure to specify "ANSI" or "ASCII" encoding in whatever application you are using to create these files.
You can use any ANSI character set you wish. The character set to use for encoding is set under the Advanced Options dialog. The default encoding character set is iso-8859-1.
ANSI is a superset of ASCII. ASCII defines the first 128 characters, and ANSI extends this to 256 characters. The encoding character set determines what symbol is displayed for each of these 256 possible values.