--- "Kurt A. Schumacher" Kurt.Schumacher@schumi.ch wrote:

Just a guess: The more retries to a certain SMTP destination, the longer the retry interval...leads to exponential delivery times.

no, it's quite sporadic: some emails are retried wihtin 10 minutes, and some were delaying for hours.

Pure greylisting is - just like blacklistsing - is useless or leads to unexpected behavior without a well-maintained and monitored whitelist for a corporate or ISP e-mail system.

it is actually a well-maintained whitelist, but of course sometimes exceptions happen...