Re: Can Exchange server and office 365 coexist?

Hybrid configuration is just a set of configurations that allows you to use the same email suffix (ie the "@currentdomain" portion of the email address in your example) in two different locations (on-premises and in the cloud). It's the email suffix that tells the world where your messages need to go (via the MX record in DNS). The email prefix (ie the "newmailaddress" portion in your example) is irrelevant for mail routing, but important in Exchange as Exchange won't accept messages for email addresses that don't exist (unless the domain is not authoritative in Exchange). You could set up a lot of the configurations manually, but it's really not worth it. 

If your MX record points to on-premises Exchange for your "currentdomain" you need to somehow configure Exchange to accept the message and then forward it to Exchange online where the mailbox resides. There are multiple different ways to do this, but using the hybrid configuration wizard is the best and easiest.