Suddenly my Outlook 2010 and eMClient email programs can only intermittently connect to my POP3 mailbox to download email. After assuming an ISP and/or router issue was the cause initially I've discovered it's Avast Free Business Security that's to blame. If I disable the Mail component then Outlook and eMClient both work perfectly. It's a very simple POP3 server, uses the standard port of 110 and doesn't use SSL encryption, just plain username/password to gain access. I'm presuming Avast 17.7.2526 is to blame as it only started recently. I've uninstalled and re-installed to no avail. Running Windows 10 Pro v1709 64-bit.
Telnet to my POP3 server on port 110 with Avast enabled doesn't work, but it does if Avast is temporarily disabled.