This a known issue for Gmail users.
When an email from a Gmail address is posted to a mailing-list, the Mailman server delivers the message to all the subscribers - and that includes the sender.
Gmail thinks this message coming in for the sender is the same as the message that the sender just posted -- so it discards it.
As a work-around, an acknowledgement email can be sent to the sender.
The acknowledgement email isn't a copy of the sender's original message, but it lets them know that their message was received by the list.
The acknowledgement email setting can be enabled by the list's administrator or subscribers (members) can do it themselves.
For list administrators
- Log-in to the list administration panel.
- Click on -- Membership Management. This will present a table -- Membership List.
- In the column -- 'Ack' -- tick the checkboxes for each of the subscribers who want to receive acknowledgement emails.
- Finally, at the bottom of the page, click the button -- 'Submit Your Changes'.
For subscribers (members)
- Go to the list's public information (subscribe) page and scroll to the very bottom.
- Enter your email address and click the button -- 'Unsubscribe or Edit Options'.
- On the resulting page, enter your password click the button -- 'Log in'. (Recover your password)
- On the resulting Member page, scroll down to -- 'Receive acknowledgement mail when you send mail to the list?' -- and click 'Yes'.
- Finally, at the bottom of the page, click the button -- 'Submit My Changes'.