It is very likely your Mac has very little to do with it. Gmail and other ISPs have limits like this. And they don't send back an email as they assume you are a SPAMer. What tye typically do is just drop the connection or issue back an SMTP 5xx error which rarely makes it way back up the network stack to an email client intact. But I'm betting on them just dropping/closing the connection. Which is why from your point of view of your mail client "it just didn't work". Mail servers, especially at larger ISPs, drop inbound SMTP connections for a variety of reasons. Mostly they boil down to "go away".

If you really want to do this a lot you'll need to look into services that allow you to maintain large lists and send them emails as desired. Many are free for list sizes like yours.

Poking around Gmail on email address limits, it seems that there is a limit of 100 email addresses per list. I did successfully send out an email to 100 addresses. I did not try for 101. This is an example of un-Mac like behavior on a Mac- getting an alert claiming invalid addresses, when a simple alert announcing the 100 address limit would have made things so much simpler.

Today I entered 109 new email addresses into my (Snow Leopard) Address Book, creating a single group. I also created a new Gmail address, intending to coral all responses to the new address. Apple Mail did not allow me to send this group mail- listing a bunch of email addresses as invalid. When I tested by sending the same message to a single email address out of the invalid ones, the email was sent. And, when I split the group into two groups of 55 and 54 each, I was able to send the entire new sub-groups. My question: is there a limitation in Mail, or Gmail, and/or Roadrunner mail, limiting the number of individual emails listed in a group? Does anyone reading this understand what is going on? I need to be able to email the entire group as a whole, so people can then reply to the entire group, not just the half-group they are in.

Thanks,
Howard Partner