Using Norton by any chance? I hate antivirus products, and Norton has a special place in my heart due to it being even more invasive than most. Over a number of years it has become apparent to me that antivirus programs cause more mail problems that any other singe thing. In the case of Norton it is something called SONAR that breaks the mail setup process.

Of course comparison to Outlook (the king of non encrypted connections) invariably points to Outlook not using connection encryption and hence working nicely. Despite the rest of the internet using it. Often poorly, but using it

So what connection security are each using? SKY recommend SSL https://www.sky.com/help/articles/getting-started-with-sky-yahoo-mail but I would disable any email scanning by your antivirus before you even start and encrypted email can not be scanned without manual additions of certificates in most cases. (with instructions on the web sites of most antivirus vendors.)

Using Bitdefender antivirus, have also used F-secure, but had same problem. Tried turning off anti virus but no different. My 2 other email accounts are outlook and GMX and they both send and receive with no problems

Phil