This isn't a question, it's just a piece of information that may be of interest to someone else in the future.
After spending a few days/sessions trying to track down a couple of problems* with my BTYahoo email address with our friends in India, one of the agents said I had to change my email password as I hadn't changed it for some time.
This I did and all appeared to be well while using Webmail on my PC. I then tried to set up my mail clients on my phones and tablet. No matter which client on what platform I tried I couldn't log in, I kept getting various error messages. Some just said 'Error' others cited a problem with username and password which I knew couldn't be the case as Webmail worked .....or so I thought
Several more sessions/days with our friends from India later, no solution. Completely different answers from each agent. The last one brushed me off by saying it's a 3rd party issue (the clients) so they couldn't help - that was annoying. Another said I would be moved to BT Mail - that was a shock after years of waiting for that to happen. One session I got passed to three agents by the end of the chat. Another was suggesting I was using the wrong settings. I don't think they knew the answer at all.
These days lots of systems are requiring the use of upper and lower case, numbers and 'special characters' in passwords so I have been doing so on all my accounts whether required or not. I had done the same with my btinternet email account.
Anyways the answer appears to be that you can't use every 'special character' or maybe it's the number of them I used, or perhaps the fact I used several in a row. I can't be ar$ed to experiment... Anyway I used just common one and all was well.
* The original problems don't appear to be fixed
Disappointing reading and something I have experienced in the past when needing help.
I thought however that BT had brought all their teams back to UK Based Call Centres - is that not so?
Are you using Apple devices by any chance? There seems to be an issue with them when using some special characters.