Aha - so AVG was the issue after all this time

- at least we now know the cause of your problem.
I was about to tell you about:
Adjusting settings for Avast Antivirus Core Shields .
But it appears your AV is less specific for its settings... so you could use other addresses for sending messages -- or you could add a POP Peeper exception (that would only apply to the POP Peeper exe -- all other security would remain intact). If you want to try using an AVG exception take a look below.
Here is a website covering a number of AV programs (including AVG) in regards to setting up "Exceptions" - so you could add POP Peeper (using the path to its executable) to solve the problem and all of your security for everything else would remain intact (in as much as only POP Peeper would be the one "Exception").
How to Add Antivirus Exclusions to your Antivirus Software (you could take a look here for the steps to do this with AVG)
I've extracted the steps for AVG for you below (found in the link above):
AVG:
1) Open AVG Internet Security user interface and go to Menu > Settings
2) Ensure the General tab is selected, then click Exceptions
3) Click the Add Exception button
4) Add an exception in one of the following ways:
4.1) Type the file path, folder path or URL into the Text box and then click Add Exception.
4.1.1) On Windows: C:\Program Files\Private Internet Access
other non-windows steps removed
4.2) Click Browse, select the box next to the file or folder you want to add to the exceptions list and then select OK
In this case (for the last "4.2" step above) you would "Click Browse" in AVG and then find the POP Peeper exe (file) and then check the AVG box for the POP Peeper exe file (next to file) and click "OK" - to add POP Peeper to the "exceptions list".
To clarify, you would be selecting the POP Peeper executable file (by locating the file via the file path to the exe - see example below).

- The typical file location for POP Peeper exe
Good judgment comes from experience and a lot of that comes from bad judgment. - Will Rogers