Mail2 should download some blacklists, contact different hosts for update (DNS servers, MX Records, SPF, etc etc).
I’m quite… confused…
I installed a 7.3 (still not updated) mailserver into a office with a Zywall 2 Plus firewall as network gateway, Windows as DNS server. Often Zywall2 was “stuck” into too many NAT connections, but was not able to stuck the internet connections of the whole office; this appliance has a 3000 nat session limit and a session limiter par host; it was configured to 2500 hosts. Internet connection was a DSL 14/1mbps
I own an old USG20W (not “VPN” version) appliance, updated to latest firmware published by Zyxel HQ.
The session limiter has a maximum setup of 8192
with the capability to setup rules for few hosts (for instance 512 as general rule but 2.048 for 4-5 hosts)
IMHO it’s strange that internet connection stuck like that.
Maybe your ISP use a ratelimiter for your connection? FritzBox do not have ratelimiter for connections, it’s just a matter of memory used by other software functions; 7490 stated “several hundreds” of connections.
If mailserver is a public mail server should be 24/7 available. If power consumption is quite too heavy, maybe you can find a bit more power efficient server (old corporate desktops have remarkably efficients PSU with capability of a couple of SATA disks)