Ok, finally found a part solution!!!
Seems other people have had this problem too.
In the maildrop README.postfix file, I found this:
Postfix, by default, will send messages to multiple recipients at a time.
Maildrop assumes that it will be receiving messages for one recipient at a
When postfix hands maildrop a message with multiple recipients, only the
first recipient receives the mail.
If maildrop is configured as a transport named "maildrop" in postfix
(which would be the normal way to do it), adding:
maildrop_destination_recipient_limit = 1
to /etc/postfix/main.cf fixes this behaviour, and causes postfix to hand
messages one recipient at a time to maildrop.
Ok, so I added the "maildrop_destination_recipient_limit = 1" line to my postfix main.cf file.
Then reloaded postfix, tested by sending email with multiple recipients, and it didn't work.
So I tried something different, turning off content filtering (spam/virus filtering with clamav, spamassassin, via amavisd).
I did this by commenting out this line in main.cf:
#content_filter = smtp-amavis:[127.0.0.1]:10024
Then reloaded again, tested, and YAY it worked! It was correctly delivering to all recipients!!!
So, why I wonder does it not work when the "content_filter" is turned on?
Only when its off?
Thats the next thing I'll have to find out.
I can't leave the content-filter off, as its removes on average 500 spam and 10 virus emails every day!