We have also used Hyper-V replication to replicate Smartermail without an HA cluster.
We have around 12,000 mailboxes approaching 2 TB of storage and lots of activity in SmarterMail handling around 60,000 emails each day and have no issue using this method, which we have been using for around 10 years.
We are using a couple of SuperMicro Xeon E servers with NVMe SSD's on the primary server and SATA SSD's on the replication server and use a 1Gbps network between the two servers, so you don't need any expensive hardware.
You can also replication failover and reverse the replication when you need to do hardware maintenance which avoids lengthy downtimes.
We use Altaro for the backup on-site and off-site rather than Veeam.