Not everything is possible. Experienced email template designers will already understand the limitations of various email clients, and will adapt their designs to that.
But even from the most experienced designers we sometimes get designs where we run into a situation which technically cannot be realized.
Most of these issues arise when we get both a desktop design and a separate mobile design, where the mobile design is for explaining the needed responsive behavior of the desktop design.
When we run into this kind of issues, we will contact you for asking how to work around these situations.

This depends a bit per customer. If you are a potential large account customer, we will be quick in offering to build you a couple of your template designs for free. Just to show you we can.
For customers finding us through our website, we offer the three free email templates for download. Check them out, see the work we would deliver.
And we ask any customer to send us some of their designs, so that we can give a proper quotation on their 'average difficult' template design.
So, to get started, contact us through the contact form. We will contact you soon!