Who issues incoming & outgoing server addresses?

Who issues incoming & outgoing server addresses?

I bought my domain at "#1".  It's being hosted at "# 2". I want Zoho.com for my mail client ("#3"). Where do I go for the incoming & outgoing server addresses? Where I bought it...where I host it, or where I go to receive my email? Do they come
from the same place or separate places?

Thank you in advance.