email split servers or split tunneling

Hey guys.  I have some domains that I have split the addresses, some on Office365, and some locally in my mailserver.  How do I configure my mailserver so it can send to the email addresses in O365. The emails are the same domain. 

I have successfully configured O365 already, it can send emails from O365 to the emails in my mailserver.  But if I try to send an email (via ROUDNCUBE/WEBMAIL) from my mailserver to an email located in O365, I get an error:

SMTP Errot (550): Failed to add recipient
"email-at-O365@domain.com"  (5.1.1
<email-at-O365@domain.com>: Recipient address rejected:
User unknown in virtual mailbox table


I have configured MX record to point to office365 as well.

Also on a side note, if I want to send an email to any other domain, how can I configure it so that it will use the office365 as outbound? If I use POP I can set the outbound and use one of the email in the O365 to send the message.  But I prefer to use the WEBMAIL/ROUNDCUBE, how can I accomplish the same?

Thank you