Interesting subject email providers.
ISP's: The majority of my email addresses are with my ISP (same one since 2005), never had a problem with them until I built my new computer last year and started using Opera as my email program.
Within a couple of months every single time I went to send an email through any of the ISP accounts I was directed to a security page where I was asked to fill in a captcha, answer security questions, and then change my password.
After numerous attempts to get the issue resolved I got put in touch with the Chairman's Office and the second person I heard from actually took the matter up although with limited success.
At one point I was told it was because I was connecting from an unrecognised IP address, as I had been using a wide range of VPN's for several years I asked them why, if that had never been a problem in the past it had suddenly become one? In spite of denying any changes being made at their end, a couple of months later the problems disappeared and have thus far not returned.
Yahoo: I have half a dozen Yahoo accounts, 2 - AU, 2 - Uk, 2 - US. The first 4 I am able to forward to my email programs, which I think is probably the reason I have avoided the login problems that a lot of people have had. All my accounts are IMAP/SMTP.
To answer a question asked above about contacting Yahoo, at least in the UK, I found this some time back although I have never used it: Yahoo UK Customer Services - 0800 0289 562.