I am designing business cards in Illustrator, however my printers use Corel Draw and some of the logos do not show up on their side. They mention 'overprinting', but I do not know what that means. 

They also say that the cards have a combination of RGB, CMYK and Pantone, how do I change all the colors to CMYK?

[![Illustrator screen shot][1]][1]

From the printer.....

[![Screen shot from the printers][2]][2]


  [1]: https://i.sstatic.net/48SX8.jpg
  [2]: https://i.sstatic.net/GCsfP.jpg