Sympa itself cannot retrieve incoming messages. Sympa receives
messages passed by postfix running on the same server.

Additionally, if postfix on your server cannot accept SMTP
connections from the Internet, you have to manage another server
that accepts connections everytime and relays incoming messages
to your server.

If POP3 is only way you can use to retrieve incoming messages, you
have to manage it by yourself. For example: First you may write a
script to fetch messages with POP3 using fetchmail and to store
them to Sympa using sendmail. Second you may run that script
periodically by cron or launchd.


https://listes.renater.fr/sympa/nomenu/arc/sympa-users/2018-02/msg00023.html