Hi Niwantha, "Communication" is the process of exchanging ideas, opinions and information. There can be different processes (plural) but each process is a set of regular steps or actions taken to achieve a particular end and is considered singular. Examples: There are several processes (plural noun) such as germination, pollination and fertilisation in the life cycle of a flowering plant. However, "germination" is singular (there is no such word as "germinations"). // The reproduction of penguins NOT The reproductions of penguins. Back to your sentence, it should be written as: "With reference to the communication/correspondence you have had with my colleague John, please drop by on Friday at 10.30 a.m. for a discussion." I hope this clears the air.

Communication can be singular or plural. It would probably sound better to just say "communication" but "communications" is also correct if there were indeed multiple exchanges (emails) back and forth between the two individuals.

In your case, communication would not be countable. You can also say emails instead of communication. With reference to the communication you had with my colleague John, please drop by on Friday at 10.30 AM for a discussion. If you are free, can you please correct my Japanese notebook? I would really appreciate it.