Like you, I use BT as my ISP (and seem to be unable to change it) BT, understandably, cannot be responsible for 3rd Party E-Mail Software (eg Outlook) and always refer me to, and insist that I use their Web-Site Email "Service."
My MS Office2010 version of Outlook is riddled with infuriating problems but otherwise works well enough. Previously I used Outlook Express (OE) but they stopped "supporting" it, but to me OE was by far the better version and behaved impeccably. As for BT, I am currently using the Ombudsman (Communications) to try to resolve my long-standing issues with BT's abysmal Customer Services. One problem you might have encountered is trying to set up your Account(s) in Outlook. If so, you are now FORCED to use an IMAP Server when entering your Account settings, as POP3 is no longer supported by Yahoo. It took BT 6 months to finally admit this - after shoving me through their Tech Support (at a Cost) but they hadn't got a clue either. (NB... Yahoo is a 3rd Party Software ) and I was told a long time ago that BT had severed their Association with Yahoo... but they are still using it, and indeed their entire Web Site is only on-line courtesy of Yahoo.
If this hasn't helped you, feel free to reply with more detailed problems you've encountered.