I am not familiar with Mirapoint's products specifically, but assuming they behave like other spam-checking appliances, I think the following is happening. If you run a mail-merge delivery with 27,000 recipients, the Mirapoint system must look for spam and viruses 27,000 times because each message is different. Even if it is very fast, which spam checkers usually are not, this is going to take a while. As a reference point, the spam checker I use needs 1.1 sec per message on the average, depending on the contents of the message. Some messages need a lot longer.

Groupmail might not be using mail-merge and might be sending, for instance, 27 messages with 1,000 recipients each, so the Mirapoint system only needs to look for spam 27 times. I assume that this must be the explanation, since your settings are otherwise correct. The last time I ran the LISTSERV HPO mail-merge benchmark, my desktop (then a one-core AMD 3800+) could create and forward over 2,000 messages per second to the SMTP server (about 8 million per hour). Of course, this does not include the time the SMTP server needs to deliver the message.

The best solution would be to bypass the Mirapoint server for outgoing mail. If you already check incoming mail for spam and viruses, there is no need to check it again when LISTSERV delivers it.

  Eric