domain MX record for subdomain? emails going to spam

I am setting up and testing a new server (linode VPS), while in the meantime running all services on my old linode VPS.

My email is handled by googlemail, so the MX records have the various *.googlemail.com and *.google.com as Mail Server with nothing set in subdomain field. Such as:
Mail Server Preference Subdomain TTL
ASPMX.L.GOOGLE.COM 10 1 day

I am running my new server as a subdomain of my old one. So I have added A/AAAA records for subdomain names specifying the new server's IP address. This works fine, apache2 works fine, no problems there.

But I am trying to set up the new server to send email using postfix. It will not receive mail since google is the receiver.

I also set up a public Reverse DNS entry for the new server's subdomain IPv4 address with Reverse DNS = new.mydomain.com

When I send email from the new server, the email ends up in spam.

I added an MX record for the subdomain, just like one of the others except with the Subdomain filled in with the name of the new server (just subdomain name), like this:
Mail Server Preference Subdomain TTL
ASPMX.L.GOOGLE.COM 10 1 day
ASPMX.L.GOOGLE.COM 10 new default

This hasn't helped. When I tried using mail-tester.com, it told me that there wasn't an MX record for new.mydomain.com.

Can anyone suggest what I should do to keep the emails sent from new.mydomain.com from getting treated as spam?

Thanks,
Evelyn