I'm getting this error from one of my contacts. It happens when I answer a ticket. The ticket email does not reach him.I use the free version, so I can not register DKIM.
What should I do?

I'm getting this error from one of my contacts. It happens when I answer a ticket. The ticket email does not reach him.I use the free version, so I can not register DKIM.
What should I do?
Enter your username or e-mail address. We'll send you an e-mail with instructions to reset your password.