Server does not recognize my recipients when sent as group email

I've just been trying to let everyone in a couple of groups in my address book know about a change of email address. There are 81 contacts in one group, and 200 in another.

Now, when I try to send the email to those groups, I get the following error:

The server "smtp.talktalk.net" did not recognize the following recipients:

And then it lists some of the addresses, about 25 for the first group and well over 100 for the second.

When I send to the 'unrecognized' address individually, it is sent through without a problem, but as part of a bulk email to a big group it doesn't work. I phoned my ISP, TalkTalk, and they assured me there was no limit imposed by them on the number of recipients, that they hadn't blocked any ports or set any spam / junk filters.

So I broke the lists down to fewer than 50 people each in multiple lists. This now works and the email is sent.

If the ISP isn't creating a problem, then why can I not send to large mailing lists? This is going to be such a waste of time if it can't be sorted out...

I saw a similar thread on the Tiger forum that didn't seem resolved. I never had this problem in Panther, even with bigger lists.

Any ideas?

Cheers.