I use the default TCP110, but my ADSL router has a NAT firewall, so the port is not accessible from the Net.

The reason I suggested changing POPFile's POP3 listen port has nothing to do with your router or internet connection; the reason is to do with how Eudora's POP3 traffic is handled within your computer.

Normally Eudora connects directly to mail servers on the internet to check for (and download) new messages:

Eudora -- internet -- mail server

When POPFile is used to handle POP3 mail, it usually acts as a POP3 proxy. This is achieved by changing the account settings in Eudora (the POP3 server and the account name) to insert POPFile into the POP3 proxy chain. Once this has been done the POP3 commands to check for (and download) new mail pass through POPFile:

Eudora -- POPFile -- internet -- mail server

If your anti-virus package is configured to scan email then the anti-virus package will insert its email scanner into the proxy chain on your computer (i.e. POP3 commands and data are now passing through Eudora, POPFile and the anti-virus package's email scanner). Most anti-virus packages do this transparently so all you have to so is enable this anti-virus feature if it is not enabled by default.

By default Eudora uses port 110 to send and receive POP3 commands and data. By default POPFile uses port 110 to receive POP3 commands from Eudora and to send POP3 results and data to Eudora. By default your anti-virus package's email scanner will scan data sent and received via port 110. So you can see that a lot of POP3 traffic is passing through port 110 (and getting handled by Eudora, POPFile and your anti-virus package's email scanner) before it gets as far as your router.

Some users have in the past reported that their anti-virus software has been scanning email messages twice when the default 110 port is used (e.g. incoming messages are scanned as they pass from the internet to POPFile and scanned again as they pass from POPFile to Eudora).

My suggestion is to use a port other than 110 for the POP3 communication between Eudora and POPFile. This requires some changes to Eudora and to POPFile.

In Eudora the "POP Port" setting is, I believe, normally a hidden setting so you may need to enable Eudora's "esoteric settings" plug-in in order to be able to see and change the POP3 server port setting. The wiki's "Configure Eudora for POPFile" page has a "Changing the POP3 port used by Eudora" section that has links to some Eudora support pages with further details and screenshots.

(As an alternative to enabling the "esoteric settings" plug-in it is also possible to edit the Eudora.ini configuration file using a text editor. If you have more than one persona (or personality) defined in Eudora and only some are configured to work with POPFile then more than one line in the configuration file will have to be changed. It has been many years since I last used Eudora so I don't have the necessary information handy at the the moment.)

As far as POPFile is concerned only one setting has to be changed. The "POP3" section on the CONFIGURATION page in POPFile's User Interface has a setting called "POP3 Listen Port" which controls the port POPFile uses to communicate with Eudora. (This setting does not affect how POPFile communicates with the mail server on the internet.) After applying this change you will need to shut down POPFile and restart it to make it use the new setting.

The wiki suggests using port 123 as POPFile's POP3 Listen Port (as it is an easy number to remember and unlikely to be used by other software). This should ensure that the anti-virus package's email scanner will not pay any attention to the messages passing between Eudora and POPFile but will continue to scan the messages between POPFile and the email server.

I have been using a non-standard POP3 listen port (either 123 or 321) for my POPFile installation for over 8 years now and have never had the sort of problems you have reported.