In the other thread about your Yahoo mail problem you said that you used a different outgoing server in the settings of the Yahoo account. Do you still use a different one, or is it the Yahoo! SMTP server?
Here's something that helped someone else who had problems with his Yahoo account, hopefully it will fix yours as well.
If you set up the account in your settings (Mail, Contacts, Calendars) with the Yahoo option, remove the account and set it up using Other, and using the settings you'll find when you enter your email address here: Mail Settings Lookup - Apple Support
Then look at the folders of the account in your Mail app. Are some of them unable to load emails?
In that case, access the account in a browser (laptop, PC), as before. Rename the folders that don't work (add "Old" to them) and create new folders with the name of the folder, without "Old". E.g.: the not working folder - Name/Old, the new folder - Name. Then move the emails from the "Old" folder to the just created one. Make sure that they are loading in the Mail app of your iPhone, then delete the "Old" folder in the account online (laptop, PC). Do this with every folder that doesn't load emails. If it works correctly, remove the account once more from Settings - Mails, Contacts, Calendars on your iPhone and set it up using the Yahoo option again.
I must be missing something. This last suggestion reads (to me, at least) as though the problem it's intended to solve has something to do with being able to send emails. That isn't the problem I currently have.
With regard to the suggestion just prior to that, you mention my using a different outgoing mail server. That was just a temporary thing trying to see what might work. It was later changed back to original settings. The settings at this moment are as shown on the Apple lookup link you provided.
You have an older phone that is probably not playing "nice", since the iOS has been updated. If Yahoo is your only email service, I would simply use the app. If I recall, it worked fine for you. If you were using more than one email service and needed to collect them all in one place I could see continuing on to try and fix this problem. If it were me, I would end trying to analyze this and just use the app. Just my 2cents worth!
Your point is well taken. I use Yahoo and Hotmail. I also have a Gmail account which I don't care much for and never use. And, as it turns out, multiple accounts are what finally led me to the solution (I believe and hope) to this problem.
It kept bugging me that one of the error messages I kept getting was asking for me to enter a password for my hotmail account while the problem itself only appeared when I tried to access my Yahoo mail. This made no sense to me. And it was compounded by the other error message stating that my connection to the server had failed.
J.A. first mentioned it and it sorta stuck in the back of my mind. Thanks, J.A.
Since my Hotmail account has a separate icon on a home page that I've always used to access it, the question occurred to me as to why there was also a Hotmail account appearing in the list of accounts shown in Settings>Mail, Contacts, Calendars. I wondered what would happen if I deleted it. Would I lose my Hotmail account altogether? I decided to risk it since my Yahoo account is my principal email account.
After deleting it I went to my home page and clicked on the Hotmail icon and it opened normally. I then closed it and clicked on the mail icon at the bottom of the page where I've always accessed my Yahoo account. My Yahoo account then opened BUT THE PROBLEM WAS GONE. I spent the rest of the evening randomly checking to be sure it stayed gone but it hasn't returned. Thank goodness.
I don't know how or why there was a second access point for a Hotmail account but it must have had an incorrect password associated with it which prompted the request for a correct one. I can't guess what prompted the error message regarding a server failure but, at this point, I don't care. I'm just happy that the problem has been solved.
Many, many thanks to all of you who contributed your time and expertise toward solving this PITA. You're great folks and I very much appreciate your help.
Glad you solved it. Microsoft, like all software providers likes to do things that probably assist when it's your default service but cause issues when it's not. Nice that we have these smart people to puzzle things out with.