Hardware requirements

It's recommended to allow all outgoing traffic for license, anti-spam and other modules to be fully functional.

Hardware considerations

Use fast disk systems; SAS RAID or SSD
The more RAM the system has, the faster it becomes (thanks to disk/database caching)
Make sure the storage disk is at least 10 GB; some optimisations (for example database checkpoint segments) depends on it

Increase overall throughput

Run globalview() or other IP reputation service in the CONNECT or RCPT TO context
Use CYREN's RPD instead (or at least before) SpamAssassin, because RPD is much faster
Instead of anti-virus engines, use DLP to block executable file extensions (even in ZIP files) using options ["stop_on_match" => true, "timeout" => 10, "recursion_limit" => 0]
On a hardware with many, fast CPU cores SpamAssassin performance might be increased by raising antispam_sa_processes
Many high-volume hosting providers need to disable SpamAssassin DNSBL lookups
If you have many listeners (mail_server__) on different ports but with the same settings (flows, etc), consider having just one listener with multiple ports (25,2525,587) instead, because the number of available threads are divided between listeners
If you run http() frequently (possibly for queries or logging), consider using the background flag to dispatch queries in the background. If that is not possible, avoid HTTPS (if the network is already trusted), and if you need encryption, install the certificate locally on the PKI page instead of relying on the ssl_default_ca option