If you are sending messages to an email group to which you are
subscribed, are you sure that you are subscribed to that email group
with the address you used to send the email that bounces? Most email
groups are set up to reject incoming emails from an address that is
not registered for the group (i.e., posts are allowed from "members
only").
For example, if you are subscribed as "abc @ something.com", and you
have your emails on that account forwarded to your "def @ gmail.com"
account. Then, even though you can receive and read emails from the
group in your Gmail account, you cannot send emails to the group from
the Gmail account because it isn't the one that is subscribed to the
group.
Many email groups also reject attachments, or emails with attachments.
Email attachments are a very good way to spread computer virii, or to
overload hundreds of email accounts with annoyingly huge emails, which
is why they are frowned upon (and rejected by) most email groups.
You might try contacting the moderators of those email groups where
you are having these problems, since your emails are being rejected on
their end.
Regards,
Andy