I don't think it's the encoding of the script that's the problem. I think you need to specify the encoding of the message in the SMTP envelope. Have a look at http://stackoverflow.com/questions/10752853/php-mailfrom-cyrrilic-encoding . This gives you an example of how to adjust the headers in general. You can do this same thing in Tcl by adding your headers after the default set of email headers.
For Base64 encoding, use ::base64::encode in EEM Tcl:
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...