Our email list is hosted by Yahoo as a Yahoo group. You need to subscribe to this list before you can post to it. Joining the C380 website does not subscribe you to the Yahoo list, you have to do this separately.
Send an email to Catalina380firstname.lastname@example.org requesting to join. I should then get an email from Yahoo to approve your subscription. After that you can post to the group. Be sure to send the email from the email address you want to use for the group emails. This is how Yahoo works, you are subscribed by your email address.