Allow / Open SMTP only to one user / domain or specified emails

Hi, For years I do not open SMTP ports (25, 26, 465, 587, in CSF via WHM panel). All MX records set to G-Suite (Google Apps) so no stress with SPAM. Now for one domain I forward all emails via cPanel to another email from other domain (domain redirected to other domain). Cannot pay a lot of money to G-Suite as long as the old domain is now redirected to new domain. So, now I'm forced to open SMTP ports, WHM always updated to latest version, CPHulk activated, Greylisting activated, Outbound SMTP connections are restricted, Prevent "nobody" from sending mail is ON, SMTP on another port (26) activated, all cpanels accounts have 2FA activated + CSF restricted access by IP or hostname, SMTP_BLOCK ON in CSF. But when I check in WHM: Mail Delivery Reports I have spam emails with status "Sender verify failed" but some of them bypass everything and are Accepted. Spammers use some generator because they send emails I do not have created, like john@mydomain.com, alice.c@otherdomain.com, office@anotherdomainofmine.com etc. My emails are forwarded correctly from old domain to new email of new domain, but for this I must to fight spam with server / all other cpanel accounts that are on G-suite, so no need SMTP open (web-scripts are solved via SMTP_ALLOWUSER / SMTP_ALLOWGROUP). My questions are: 1. It is possible to open SMTP only for one cpanel account? 2. It is possible to open SMTP only for one domain or certain emails addresses? Like open SMTP only to a@domain1.com, b@domain1.com, c@domain1.com?