Greylisting and broken MTAs
It's still amazing how many broken MTAs are out there that don't perform a resend within an appropriate time period. Greylisting is set to 60 seconds and a second delivery attempt will already whitelist but e.g. the following MTAs are too broken to retry delivery (additional to those already listed in the Debian supplied whitelist_clients:
mail.gw.deloitte.at[193.22.85.34] ip34.juprowa.at[213.129.253.34] mail.nextra.at[195.170.70.67]