The size of the message you are trying to send exceeds a temporary size limit of the server. The message was not sent; try to reduce the message size or wait some time and try again. The server responded: 4.5.3 Error: too many recipients.
We set this in main.cf:
smtpd_client_recipient_rate_limit=0
We didn't specify it before and the default is said to be 0/unlimited and it still didn't change anything.........