We had a small local group for some short time and were using a bulk remailer.
This only for around 20 people
The recipients that had problems were those on Gmail. The mails or rather worse still just some of them were not getting though.

We abandoned the remailer for sending them out from a personal home broadband type ISP based email account.

I still do an occasional circular from my own email account for 60 people - done by the usual bcc system to hide address. At one stage hotmail receiving address had an 'issue' with receiving these bcc but not with normal cc emails......then the problem disappeared.

I have a hosting package with TSOhost, and using the supplied package (albeit via a 3rd party) for mailing lists, found that Hotmail and AOL were accepting mail, then throwing it away. Perhaps this is a similar thing

From looking at the error when I could on her phone it seemed to be the gmail SMTP server rejecting the sending of the email rather than the gmail recipient rejecting the receipt of the email - but it was getting late at night and I didn't have time to do much analysis.