Hey Romain,
The notification templates and settings unfortunately does not accept HTML.
It will be added in RTF.
A means to do so however, I can't off the top of my head think of one that is available to be done on the ESA.
EDIT: Yep, it's not possible as the ESA is RFC Compliant, and returned (notification) emails cannot be structured with HTML and must be encoded in MIME type of text.
The only resource that I know accepts HTML code for normal email flows is the Disclaimer but this merely stamps a heading or footer on the email.
A non-ESA enabled optin is, you can redirect emails from your ESA to an account which will have an auto-response feature which allows that mail server to reply with a HTML-formatted message, since these systems allow the mail administrator to design the email in HTML and have it sent every time a new email arrives to that account.
Regards,
Matthew
