“You getting this message” errors

My client is getting these fairly frequently:

You getting this message because an error detected while delivered your email.
For the domain: https://www.schmidtcustomfloors.com
The log to paste when you open a support issue:
Invalid “Reply-To” e-mail address “[email protected]”

She’s connecting with Post SMTP to a Rackspace mail account. Everything else with mail seems to be working fine, is there a way to stop these errors/error messages? Forms in this site are using Gravity Forms.

The page I need help with: [log in to see the link]