I began receiving the "Sending of password user <user> did not succeed. Mail server incoming.verizon.net responded: Authentication failed." It then prompts me to retry the password. Similarly, outgoing mail can no longer be sent. I am prompted to re-enter the password, then get "Sending of message failed. Please verify that your Mail & Newsgroups account settings are correct and try again."

I have Thunderbird set up to download email from three accounts, two POP servers through GoDaddy and one POP server Verizon. The Verizon account is also the designated outgoing server. The two GoDaddy accounts can still receive email, but from one day to the next, the Verizon account stopped working. I began receiving the "Sending of password user <user> did not succeed. Mail server incoming.verizon.net responded: Authentication failed." It then prompts me to retry the password. Similarly, outgoing mail can no longer be sent. I am prompted to re-enter the password, then get "Sending of message failed. Please verify that your Mail & Newsgroups account settings are correct and try again." Here is what I have done in terms of trouble-shooting. 1) The password and username are correct. I can access webmail, and also send/receive via my phone, iPad and my laptop. 2) The server settings are correct. They did not change from one day to the next. I verified and re-verified them. Additionally, they are the the same across the different platforms, especially the laptop which is using an identical configuration of Thunderbird. 3) I do not believe this is a problem on Verizon's end because it works on every other platform. This seems specific to this machine and this configuration of Thunderbird. 4) I uninstalled and reinstalled Thunderbird, even though I didn't think that would do anything because the profiles are untouched in a reinstall. 5) I cleared the cache of both Thunderbird and Firefox, deleted .tmp files, restarted the browser and application, restarted the computer. I deleted all the passwords retained by Thunderbird and re-entered them. 6) There was a windows critical update the day before, which I rolled back through system restore. 7) There was a Thunderbird self-update to 31.7.0 the day before, but I am unable to undo that. (FYI, using Windows 8.1 pro) 8) I use Windows Defender for Antivirus, I tried enabling/disabling it. I also made sure Thunderbird is checked and allowed to access the internet through the firewall. 9) Also, weirdly, when entering server settings for the account in question, it has begun throwing another error, "The Local Directory path 'C:Users\<user>\AppData\Roaming\Thunderbird\Profiles\<profile.default>\Mail\incoming.verizon.net\' is not suitable for message storage. Please choose another directory. This is also new, and from everything I've read, that is a perfectly acceptable location. So my guess is that somewhere, some file was corrupted either by the Windows update or the Thunderbird update, I just don't know what that file might be. This is really driving me nuts and I would love some help.