I have tried to send an email with a pdf attachment(790k) to 5 main
groups that I have created. Each main group contains other groups with
a total of 100 recipients in each main group. They have been sent but
have been returned with the message ?failure delivery? because of:-
<@smtp809.mail.ukl.yahoo.com>:
216.145.48.35 does not like recipient.
Remote host said: 550 5.1.2 <""@smtp809.mail.ukl.yahoo.com>...
Unsupported mail destination
Giving up on 216.145.48.35.

All five emails returned had the same message repeated many times
which seems very strange. I would really appreciate any helpful advice
please on how to send these out successfully. Is there a limit to the
number of email addresses that you can send to with one email?

Are you sure all adresses are valid?

YOu may want to check with Yahoo.  Yahoo has very specific
requirements when sending bulk emails ( to hamper spammers ). I know
from past experience with their economy hosting, you can only send out
250 emails per 24 hour period using php mail function.  They also
limited how many "addresses" could be in the header of the emails. 
This caused one of my clients some headaches.  If I were you, I would
check with Yahoo about the problem.

Regards,
Jack 
<a href="http://klicktheweb.com">KlickTheWeb</a>

Best OSX tip for this one would be to use an application called Max
Bulk Mailer from Maxprog..http://www.maxprog.com/MaxBulk.html

This application allows you to upload lists and send html and text based mails..
it also allows you to set how the mails are sent so that you can send
them in small groups so as to not upset your ISP!
It also checks subjects and mail content to see if the mail will be see as spam

You can download a trial version the full version cost $49..worth
every penny just for the spam check funtion alone..

It supports attachments so you can send pdfs with it.