This looks nicely polished and I might pay for this since it provides some real value but it has the same problem many other front-end SaaS products have - it hijacks the end user.
What I mean is that I can't use it without sending my users to your site. For example, I use sendgrid for marketing and transactional mail. I think our bill fluctuates between $200-500/month. I would consider using Sendgrid's WYSIWYG editor (even though it's not very good) but I can't do it without:
- sending my users to their website,
- managing separate accounts/permission,
- breaking continuity with my product,
- storing potentially private attachments offsite,
- loosing any integrations I could provide
What I would like from a product like yours is to be able to use your frontend as a javascript embed without the other clutter. i.e. a hosted library rather than a full product.
Your embed would have to define some sort of plug-in API for uploading images/attachments and browsing existing ones and then your clients would have to provide end points for those APIs.
I would then request the completed HTML after editing and handle all the sending and related issues myself (through my vendor like sendgrid).
I know building hosted javascript embeds isn't as sexy as a full SaaS offering but there is definitely a market for it. Lots of people sell products that send email - you could sell to those people.