Use an email that doesn't get blocked
I've had far too many clients of ours say they did not receive the email, usually because their servers are blocking an email address like email@example.com, thinking it is spam.
A very small percentage of emails are caught in recipients’ spam filters, regardless of the outbound email address, message content, or email server utilized. While this issue is very difficult to eradicate all of the time, our team continually monitors and adjusts email infrastructure to achieve as close to 100% deliverability as possible.
I have the same problem too which really making me hard to continue using rightsignature.
Some of my client may have missed out the email as they expect to receive the email from Me rather than right signature. And it does not make sense for the client to see the email is send by right signature other than ME. Some feedback from my client are they initially thought of just another spam mail and ignore it.
As a one man freelance designer, my suggestion is either add just the email customization to Personal plan (and add a little footer at the bottom of the email say support by RightSignature), or had a new plan which is between Personal and business.