Hi Ram,
1.What do you mean by compatible, what are your requirements.
Exchange can talk to any Non-Exchange servers using SMTP, like it talks to Gmail.com, yahoo.com or Lotus notes.
You might also have SSL\TLS support connectivity as well if your Zimbra server supports it. I don't see anything additional to what you would need on a non-exchange server.
2. Yes you can have send/receive connectors for Exchange, its nothing but a service listing at a specific port of the server IP. This is very similar to other mail servers that do, eg. again Gmail.com and Yahoo.com server listen on SMTP 25 for incoming
emails. Hence it would be there in Zimbra as well, but might be called someting else. Like Ex2003 called it SMTP virtual servers.
Please ask Zimbra specific questions on their forum.