I want to be clearer. Sorry about that. He is trying to set up his hotmail email address and it is not accepting his correct password.

Is the account POP or IMAP? If you have followed this guide https://support.mozilla.org/en-US/kb/thunderbird-and-hotmail you will have seen the paragraph at the bottom about what to do if you get a password error when trying to connect with POP. Have you done what it says? The Hotmail server settings are here: http://windows.microsoft.com/en-gb/windows/outlook/send-receive-from-app You will see they are different depending on whether you have IMAP or POP.

Yes, I made sure to set it up as imap, just like I do for my own hotmail accounts under Outlook 2016. Everything was entered the same way as I did with Outlook. It's not recognizing the password. I've checked, double checked and triple checked everything.

If you have actually created the account, but it's not working: Confirm that you have correct info in Account Settings > Server Settings check: User Name = full email address.

Also: Outgoing Server (SMTP) select the server in list check: User Name = full email address.

You could remove and retype making sure there are no blank spaces before or after and no typo errors like commas instead of full stops.

Also, please confirm that imap forwarding has been selected in the webmail account. It might be auto selected, but some servers need you to select it first inorder to connect.

Yes, I made sure of all that before continuing. I set his account up the same way I set up my accounts for my Outlook. Everything was the same. Email address was fine, no spaces, no mistakes. Password was typed in, re-typed, and typed a third time for good measure. Made sure the advanced section was all set up properly. And still it didn't work. My dad doesn't like like interface online so I suggested Thunderbird because I had used it and I really liked it. I'm not understanding what the problem is this time around.

Have you checked saved passwords? It just might be an old password still there. Tools /options / security / password / saved password

No it's not that either. I appreciate the help everyone but I'm no closer to solving this. It's baffling me to no end. I'm guessing it's a Windows 10 issue as that's what he has installed, unfortunately. I know Thunderbird was working fine on Windows 8 and 7 before it for me, but it do sent seem to like Windows 10. I don't know, I'm just grasping at straws here. At this point I feel it's easier just to buy Office 2016 for him and let him use Outlook since I'm having zero issues with it registering email accounts. I don't know. I'll keep looking for another email client. Anyone have any suggestions?

I'm pretty sure I have Windows 10/Thunderbird/Hotmail all working together and so I'd say I have high confidence it can work. But that's on another computer that I can't get to right now, and when I do get to it, I'm expecting to merely confirm settings you already know and have tried.

Isn't the built-in Microsoft email client the obvious no-brainer go-to client to use here? ;-)

BTW, "wrong password" is an unhelpfully specific warning message that can actually mean any of a plethora of issues, involving ports, security settings, authentication and interference from personal firewalls and other security software.

Well here I am on Windows 10. I'd forgotten how fugly it is. Too much flat and white. And the fonts! Gah!

It turns out I already had two instances of the hotmail account. I guess this was allowed to work because they use different server addresses. I keep this particular account using POP3 just as a point of reference.

I have also just added it as an IMAP connected account, no troubles whatsoever. Here are my settings:

 Application Basics

   Name: Thunderbird
   Version: 38.7.0
   User Agent: Mozilla/5.0 (Windows NT 10.0; WOW64; rv:38.0) Gecko/20100101 Thunderbird/38.7.0
   Profile Folder: Show Folder (Local drive)
   Application Build ID: 20160310150025
   Enabled Plugins: about:plugins
   Build Configuration: about:buildconfig
   Memory Use: about:memory

 Mail and News Accounts
 
   account9:
     INCOMING: account9, , (pop3) pop3.live.com:995, SSL, passwordCleartext
     OUTGOING: smtp.live.com:587, alwaysSTARTTLS, passwordCleartext, true

   account47:
     INCOMING: account47, , (pop3) pop-mail.outlook.com:995, SSL, passwordCleartext
     OUTGOING: smtp-mail.outlook.com:587, alwaysSTARTTLS, passwordCleartext, true

   account59:
     INCOMING: account59, , (imap) imap-mail.outlook.com:993, SSL, passwordCleartext
     OUTGOING: smtp-mail.outlook.com:587, alwaysSTARTTLS, passwordCleartext, true

Take your pick…

No, I don't really have 59 working addresses. This is an historic accumulation and also includes a number of newsgroups and chat accounts.

Yes I tried those settings. Remember, I did use Outlook settings which were identical to Thunderbirds. As for Windows 10 built in email client... you must be joking! Lol I used that for about a minute when I had Windows 10 installed and it was a complete joke and a waste of time. It hardly ever got email, hardly ever worked right, and the settings were rather light for an email client. Now I have used Windows Live Mail but they stopped supporting that which is understandable. I was thinking about giving Mailbird a go again for my dad. Downside is you can only have one or two active emails with the free version but my dad only has one so it will be fine for him. I'd much rather be hands on with his computer instead of remotely accessing it but those are the drawbacks of us living in separate countries. I can't get to him as readily as if I were in the States.

By far the most likely issue will be anti virus /Firewall.

My observation is those with less computer skills tend to use Norton. Those that use Norton have huge issues with every update because of the way their firewall blocks "unknown" software. Often the difference between say outlook working and Thunderbird is that the anti virus already known about the version number of outlook.

Also e3nsure that two factor authentication at outlook is disabled. They encourage folk to use it, but do not explain the application passwords required if they turn it on.