POP3 is about receiving, so we can ignore all the sending bits.
How are you receiving emails? The choices are:
- via mail-receiver Configure direct-delivery incoming email for self-hosted sites with Mail-Receiver and Admin - Settings - Email -
manual polling enabledis set - via POP3 Set up reply by email with POP3 polling and
pop3 polling enabledis set
If you have both configured, then maybe the POP3 settings are obsolete. You could try if simply disabling pop3 polling enabled does the trick.
Also, you can check /logs on your server for any clues.