this would seem quite obvious, but it is still not working :)...
I have set the general default, the context for the project as well as my own user profile to send Text email. Still it becomes an html email. Even if I switch to Text while composing the e-mail. (Default outgoing mail format for the jira instance is html).
A side effect of this is that the html-code is shown in the Comment created on the JIRA issue when sending the e-mail (since we plain text renderer in the comments).
Example from a Comment:
Every team in the world is unique, and so Atlassian believes that each and every team's best way of working needs to be molded to their unique circumstances – ...
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