In the meantime, I sent a message from a gmail account to my personal gmail account (which is configured to forward all messages to my personal email address on my own server).
If I disable milter-greylist, the message gets forwarded.
If I enable milter-greylist, the message never arrives.
In addition, there's no entry at all in the milter-greylist.log file when it does fail ( I was doing a tail -f on it).