Correspondence refers to historical communications with your customer.  See Correspondence explained to see how it is used.

Any emails sent by the system are recorded in correspondence.

One email will be visible from multiple places in the system

Sending an email is easy

How is the list of email addresses I can send to generated?

Note you can also simply record correspondence like phone calls, letters, inbound emails etc in the system as well.

Note that if your customers and suppliers are not receiving your emails - you may need to do some more setups - see email section in Starting on SaaSplications