After months of frustration – having our outbound emails randomly fail for distribution groups (alias lists in Courier MTA terminology) – I found the cause.
Our SMTP smarthost doesn’t allow multiple “RCPT TO:” commands in an email header.
By default, Courier batches up all emails forwarded to the same domain name. There’s a switch to turn this off: http://www.courier-mta.org/courier.html
This file contains one line, containing a single number. This number specifies the absolute maximum number of recipients for a single message. If the Courier mail server receives a message with more recipients, the message is duplicated as often as necessary until each copy of the message has no more than batchsize recipients. If batchsize is missing, it defaults to 100 recipients per message.