Success! (sort of). Before deleting my Yahoo account as you suggested, I instead turned off it's mail function in Settings, rendering the account inactive. I then closed and re-launched Mail from All Inboxes, and it defaulted to All Inboxes. Same when I cleared Mail from my multitasking list. I then closed Mail and cleared it from memory, and turned my Yahoo mail function back on, and all was still well - it launched into All Inboxes. Great!
However, there is now a different problem (which I suspect is not a problem, but an oddity of the iOS). This might be more easily understood if I bullet the different scenarios:
1. Close Mail from All Inboxes and re-launch. Mail re-opens in All Inboxes.
- This is true whether you clear it from the multitasking window.
- This is true if you restart the iPhone.
2. Close Mail from the Mailboxes screen (the screen that includes a list of mailboxes and accounts). Two things can happen:
- Close Mail from Mailboxes screen. Clear from multitasking screen (or not). Mail re-lanches to the Mailboxes screen.
- Close Mail from Mailboxes screen. Restart iPhone. Mail launches into Yahoo's inbox (the first account on my list of mailboxes/accounts).
So, it appears that there is a cache that stays active as long as the phone is on, which remembers which window you closed from, whether you clear Mail from the multitasking list or not.
The oddity is that, after restart, Mail launches into the inbox of the first account on the list, regardless of which window you were in when you closed Mail. It is this behavior that I will bring to the attention of Apple via the feedback link, because this is illogical and undesirable behavior.
Thank you MiFFMaN, I marked your reply as helpful. Disabling Yahoo must have re-written the .plist file, solving my original problem. However, I don't see any way to get Mail not to launch (after iPhone restart) into the inbox of the first account on the list.