As with all Proxmox products - PMG is 100% AGPL licensed software - so my short recommendation is - just install it and give it a spin
Regarding your questions:
throughput of 600,000 letters per day;
We haven't done any throughput tests recently - but this post should still be valid:
https://lists.proxmox.com/pipermail/pmg-devel/2019-November/000764.html
Additionally as usual with SMTP - you can quite easily spread the load across multiple PMG instances using DNS round-robin.
the ability to adjust the balancing;
Not sure what you mean by that - but if you want to send different percentages of mail to different PMG instances then this is not possible out of the box (and seldomly necessary) - Balancing/multiple instances in SMTP can be implemented with multiple DNS records quite robustly
If you still need to do other balancing (why would that be?) - then you can always use a TCP/SMTP loadbalancer in front of PMG (haproxy, nginx maybe) - which increases the complexity of the setup and adds another point where you need to consider redundancy
. Title equals/not equals; header("From") == "@x55.ru"
There is regex matching in the filtering system
ii. The recipient is equal/not equal; (rcpt-to == "^
XXXX@x5.ru$")
This would either be a What filter (if you mean the 'To' header) or a Who object (if you mean the envelope sender)
However currently negation is not implemented in the rule-system - you can work around this to some extend by creating 2 rules with different priority (one with the matching object and the desired action, one without the matching and the action if it does not match at lower priority)
There is a patch-series, in the making which aims to add negation though on the pmg-devel list.
For completeness sake - also the links to the reference documentation and the getting started page in the wiki:
https://pmg.proxmox.com/pmg-docs/pmg-admin-guide.html
https://pmg.proxmox.com/wiki/index.php/Getting_started_with_Proxmox_Mail_Gateway
I hope this helps!
