The simplest way to overcome this lockdown is to disable the firewall during the SMTP submission process. This might not be suitable for enterprises with stark security requirements, but may be reasonable for some other people, specially if your host is inside a controlled LAN segment.
We will launch an update containing that fix in short, just get to the URL https://33hops.com/xsibackup-vmware-esxi-backup.html in some days to download the new version.
Still, you will just be able to perform a basic Vmkfstools backup on ESXi 7.0.0