How to set up domain on remote CPanel to send mail from droplet

I am using an Ubuntu LAMP droplet. I set up an A record to point a subdomain to my droplet that runs a cron job. I would like cron to send an email to my gmail. Since I set up the A record for sub.domain.com, I cannot set up a subdomain on cpanel, therefore I cannot create an MX record for that subdomain. If I get rid of the A record and set up a subdomain, it does not route to the droplet.

I set up Postfix and cron tries to send emails, but they don’t go through.

How do I set this up.

Thanks!