News letter not showing html
Using Jamroom
We use the newsletter manager here with Mailgun, and I know it works. Note that the newsletter manager does not care what the email delivery system is - it just sends email.
1) Make sure the mailgun module has been installed on your system
2) make sure it is active, and has been configured with your proper API keys
3) make sure it is set as the ACTIVE email system in ACP -> Email Support -> Global Config -> Active Email System
4) go to ACP -> Email Support -> Test Email and send yourself a test email. If it works, you are set - if not, your Mailgun credentials are probably wrong.
If you run your own server, I would recommend setting mailgun up like this instead:
https://www.jamroom.net/brian/documentation/guides/1312/set-up-postfix-with-mailgun-for-reliable-e-mail-delivery
as that will setup Postfix to work with Mailgun, and then you do not need the Mailgun plugin (you just use the default PHP emailer in the Active Email System).
If you don't, and are on shared hosting, the mailgun plugin works - just know that it is sending over a web API which is not going to send as fast as the local SMTP server. So with that, I'd recommend going into the Newsletters -> Global Config and make sure the newsletter rate is no more than 5 per second.
Hope this helps!