As far as I can gather from the documentation (mainly regarding the sendMessageInHTMLFormat="true" option in web.config) it appears that auto-responders can only send an email in either plain text or HTML but not both?
Is this the case?
Essentially, we would like to be able to send it in both for accessibility reasons.
Anyone come up with alternative solutions to this issue? (essentially you need to be able to edit the email header to allow for Multi-Part MIME, which from what I can gauge EmailTemplateActorModel can't do?).
I think if you wanted to support multi-part emails, you'd need to create your own version of the SendEmailAfterSubmissionActor. You could base it on the Episerver implementation and override the "Run" method (as the SendMessage method is private) though you'd have a bit of work on your hands as you'd need to handle things like replacing placeholders in the email content.
There are a couple of alternatives which spring to mind. The simplest would probably be using a service like Mandrill (MailChimp's transactional email service) as your SMTP server. As I recall, you can set that up to automatically convert the email to a multi-part format as it passes through (it can also do stuff like adding tracking if you're that way inclined).
Another option might be to use the web hook actor to call something else which will send the email such as an azure function, logic app or maybe even a web API controller in your solution. If you want to maintain the flexibility of the solution though, that might end up being a similar amount of effort to creating your own SendEmailAfterSubmissionActor and would be more prone to errors from editors setting up forms.