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

How long was it previously working? How long has it now not been working?

sorry about the delay in replying My sky/yahoo account works perfictly in outlook (windows 7) my problem is trying to set it up in thunderbird all the user settings ive put into thunderbird ive copied from outlook My username and password are correct, I can verify this by logging directly into sky/yahoo email account direct. When i put all the settings into thunderbird I get the message credentials incorrect I recently put my email onto outlook on another computer (windows 10) using the settings and credentials from the other outlook accout, it all works perfictly first time I have 2 other email accounts in thunderbird (a gmx and an outlook email account) they both work perfectly.

Is two factor authentication turned on ? (you are required to get a code sent to mobile/another email etc) I know other mail providers have fairly recently required it. If so you will need to generate an App password rather than the password you normally use to login. There will be details on how to do this in the provider (Sky's website).

Also can the windows system resolve the hostname of the POP3 & SMTP servers by DNS ? VPN running can cause this to fail. If you have one try checking with it on & off and compare the results. If 2 factor auth is on, the Sky system may require you to enable DNS over HTTPS in Thunderbird.

two factor authentication has not been turned on. All I use is my username/email and password if I log into my sky/yahoo email account directly When I set up my outlook and GMX on thunderbird I never used two factor authentication and they both work fine When I set up my account in outlook on the windows 10 pc just used normal authentication the same as when logging in direct to the account in sky/yahoo Tried setting up in thunderbird with VPN on and off, still get credentials incorrect, both other emails work fine VPN on or off

two factor authentication has not been turned on. All I use is my username/email and password if I log into my sky/yahoo email account directly When I set up my outlook and GMX on thunderbird I never used two factor authentication and they both work fine When I set up my account in outlook on the windows 10 pc just used normal authentication the same as when logging in direct to the account in sky/yahoo Tried setting up in thunderbird with VPN on and off, still get credentials incorrect, both other emails work fine VPN on or off

Can you try enabling DNS over HTTPS in Thunderbird to see if that solves the issue. (I did a step by step of how to, in this post, screenshots at the bottom of the post. https://support.mozilla.org/en-US/questions/1454425)

This post https://uk.help.yahoo.com/kb/sky/SLN4724.html appears to imply that an app password needs to be generated. So you will need to login to sky/yahoo and generate this password and then use your username + the app password given to you , in the login info for the server for Tbird on both the POP3 and the SMTP.