MX records to set at another registrar?

I am possibly confused as to how this works. I have hosting with inmotion, and inmotion hosts and is registrar for my picturestodigital.com website. However, i have an old site that I have parked and pointed to picturestodigital.com called iconicjohn.com. I want to set the email addresses for that domain to be handled by inmotion's server, rather than the current yahoo email server. I think that I need proper MX records to enter in at yahoo, who is also the registrar for iconicjohn.com, to make sure emails go to inmotion. I already set the DNS name servers to inmotion (just a few minutes ago). But I do not know the correct info for the MX record--or do I need to even worry about that? Thanks and I hope my question makes sense, because I barely know what I am talking about.