Outlook for Windows aka New Outlook is not currently functioning with TalkTalk Mail. Microsoft do not allow settings to be changed.
I see that Microsoft advice to another member is "New Outlook is still under development...won't be able to use Tiscali/TalkTalk email through it. So either buy Microsoft 365 in order to get the old Outlook, or just use TalkTalk webmail."
You can of course use TalkTalk Mail webmail but you can use alternative email clients like Thunderbird or BlueMail and I have used both of these.
