Feedback for Sign Up Form, UI/UX
Yes, here is some feedback
Onboard was very bad in the segment of signup. I wrote a ticket for this. As a web developer can say signup form is really, really badly done, you must spend more time to make a sign-in process much better. My suggestion is that you first create an account, do not take people's credit card on the first screen. You first want to capture their name and email, so you can have them in your database for marketing. Then on the second screen to complete the signup process ask them for all necessary card data. And please, make sure form and buttons work right and form validation especially since that is the biggest problem for now. Regarding other onboarding stuff you need to better explain what domain people should use, that is the confusing part. I was always using EMAIL.domain.tld (subdomain) because I got that from your manual, but 1 year later I realized I wrongly interpreted your manual.
Just fix UI a bit, make it a bit more spacious and modern. Make everything in "flat" design, move submenu and logo in the navbar and make few changes, not complete redesign, because it would make customers crazy :) But just small cosmetic changes (Even we can help you with that, we are doing design as well)
We strongly recommend Mailgun to all of our clients. We especially love it because you are not trying to be all in one solution to have email builders, automation and the rest of the stupid stuff. You are transactional email service, and we, developers, love that and easy integrations. Just stay like that. If you need money, just charge us 1-2$ per month for the low quantity customers (That's for sure significant amount of money per month) just don't try to be all in one and too confusing.