How to set up different MX records for outbound and inbound email?

I would like to use my own outbound email server Haraka with Node.js and Google Apps for inbound mail.

Now my domain registrar gives one click Google Apps setup, however I’ve already set up my own outbound server and it says:

Caution: The main domain name (www.)mydomain.com is 
using MX : mail.mydomain.com already.  If you want to add Google Apps, you need create a subdomain, 
e.g. xyz.mydomain.com or remove MX : mail.mydomain.com. 

So how can I implement both correctly with SPF and DKIM records without any interference?

P.S. I’m very new at this topic, sorry if my question feels a bit confusing

MX records are not used for outbound mail, only for inbound. if your current mail. domain.com server is outbound only then delete your current MX record.

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