Question : DNS question regarding e-mail

I have received an e-mail from someone that we exchange e-mail with.  Our e-mails can not be delivered to them and he is telling me that I need to make a DNS entry, such as The one shown below.
He says I need to add      ------                     mail       IN           A             x.x.x.x

How do I do this?  I'm used to seeing----     mail          A            x.x.x.x  but not    mail    IN    A     x.x.x.x

Answer : DNS question regarding e-mail

it is the same, since IN is optional.