Custom HELO hostname

I’ve been experimenting with CyberPanel and I’m liking it so far.
I’ve put myself in the following situation:
My CyberPanel is set to operate as a webserver and mailserver for site1.tld
I’ve asked my ISP to set the PTR record to mail.site2.tld, because site2 is more important than site1.
Site2 is not yet hosted in CyberPanel, it lives in the cloud as of now.
I’d like the mail from site1 to be delivered correctly (gmail is marking them as spam).

The SPF record for site1 says “include:mail.site2.tld ~all”

How do I set the CyberPanel HELO to mail.site2.tld without moving site2 to CyberPanel?
It looks like I need to create a site2.tld DNS zone or website in CP, but I’m not clear on how to manage certificates (if needed) and set the HELO.


I tried to add a mail.site2.tld website in CP. I then generated an certificates for mail.site2.tld in SSL > Hostname and SSL > Mailserver. It looks like the HELO is fixed.
While testing towards Outlook I’ve been notified that Spamhaus has blocked my site1.tld, so there’s that one to keep in mind too.