Hi, 
could you please check plain text version of your 
Forums -  New post template (
Site Manager - Development - Email templates)?
Could you please try to reinsert its content using notepad (insert the content into notepad, copy it from notepad and insert into Plain text version textbox) to make sure, there are really only plaintext characters in the textbox?
Which version of Kentico 
CMS you are using and in which mail client you have received such email?
Thank you for information.
Best regards,
Ivana Tomanickova