Sendmail, simple mail list problem

[Alan Curtis]

I have set up sendmail to use my ISP outgoing mail server as a relay 
for mail from my server using the instructions I found here

http://www.leo.org/~barner/freebsd/articles/mailsetup/article.html

I use dyndns so that everyone can find myserver (as 
myserver.dyndns.net). I can send and receive mail on my server.

I am also using a simple mailing list with about 15 subscribers that I 
set up using a mail alias
(i.e. with the line 'mylist: : include:/usr/home/alan/lists/mylist.txt')

I can send mail to anyone on the list by sending mail to 
mylist at myserver.dyndns.net from any of my email accounts and from my 
account at my ISP (call it myaccount at myISP.com). Other users have also 
successfully sent mail to the list. However, a subset of the users also 
use my ISP and their mail to the list gets bounced by the ISP server.

The error message in the bounced email is

553 You are not authorized to send mail as <otheruser at myisp.com>
501 5.6.0 Data format error

So <myaccount at myisp.com> and <myotheraccount at mywork.com> can send mail 
to the list but not <otheruser at myisp.com>.

Any suggestions on how to fix this?

Thanks

Alan