Looks like native COI compatibility on the server makes life easier for a client, and adds advanced features as opposed to restricting some servers from the party:
>"A COI client should check for COI support from the server first. it will CAPABILITY after having logged into the IMAP service. If the returned capabilities list contains the "COI" capability, then the client does not need to filter messages on the client side. It will also have more advanced options at its disposal, see the server side specifications for details."
The requirements are just an IMAP (or JMAP) server that conforms to the IMAP RFC:
>"The IMAP server may or may not be COI-compatible. The IMAP service MUST be compatible with IMAP v4rev1 as defined in RFC 3501, which is the case for almost any IMAP service in production." An SMTP service for sending messages. Sending services are also called Mail Transfer Agents (MTAs)."
followed by
"After checking for server compatibility..."
Does this reduce the target pool? How much?
Also, how many people still use POP?