The answer is, in fact, very simple.
These eicar test messages have not been scanned by avast at all due to the default settings of avast.
Unfortunately acegap has not been able to respond fully to the requests for information that we have put.
SSL is not involved in any way here and acegap has finally told us (in a round about way) why the messages are not being scanned. That's why I asked details on how the message was delivered and for a screen capture of the message source ... which we did not get. It would have shown that the X-Antivirus headers (inserted when avast scans a message) would not be there.
acegap used a well known eicar test site to send emails to his email account on Yahoo.co.uk.
Yahoo does not scan messages as they are delivered to the Yahoo message store. Yahoo scans the messages when the user accesses the message from the message store either using the web interface or via POP3 if the user is allowed that access. In either case acegap would not have been able to get the eicar virus delivered.
Instead, acegap uses YPops to deliver Yahoo mail messages to his Thunderbird client as a POP3 stream. YPops and other similar programs (MrPostman, FreePops, the WebMail extensions of Thunderbird)) all perform this conversion by http accesses to the users mail account in Yahoo. It accesses the raw messages in the Yahoo message store, converts the message to a standard POP3 stream and delivers it to the mail client (any old client) and, in doing so, the scan performed by Yahoo is avoided.
YPops (and the others mentioned) all run as a local proxy (any bells ringing yet?). The user specifies localhost as the server and can define to YPops which port will be used (the default for Ypops is 110) but acegap told us that initially port 111 was being used.
It is the default setting in the avast Internet Mail server to ignore all local communication. All acegap needs to do to get these messages scanned by avast is to go to the Redirect tab of the Internet Mail scanner and uncheck the "Ignore local communication" box and these messages will all be scanned by avast. With the proviso that if a non-standard port (like 111) is used then that port needs to added to the POP port box in the same tab.
I first came into this forum two years ago with the same question, others have followed. There is still precious little help for anyone with the same question in avast.
Before I came here I was testing out AVG. While I was in their forum I was asked to write a post which is still a sticky at the head of their mail forum. If avast has somewhere to put a description for users of these 3rd party Webmail to POP converters and how to make them work with avast then I will be happy to put something together for review by the team.