Outbound email from Stat is HTML only. Inbound email can be text as long as the email server sends the mail appropriately. (mail servers send both html and text format to Stat even though the email itself is just text). If a site would like to modify the outbound email, here is the way to do that.
Outbound emails use \report\business_rule_report.jrxml.
This is a JasperSoft iReport file. (v4.0.1 for 5.6.x and v4.7.0 for 5.7.x)
The user can modify this report if they wish. Be sure to remove any *.jasper files from \report\ or the report won't recompile.
Also they could put the Central Agent into DEBUG and send some emails (fire some business rules, click email from a CSR or send an approval email). The server.log will show the full content of what is being sent.