My OS is Windows 10, no anti-virus software

So that means you're running Windows Defender. It's recommended to create an exception in Defender for the Thunderbird profile folder. https://support.mozilla.org/kb/profiles-where-thunderbird-stores-user-data

Try to set 'Authentication' to 'Normal password' for both, incoming, and outgoing server. Is there any difference?

@christ1 Thank you for the reply. I disabled Windows Defender and also tested with opening an exception in it for the profile folder, but the issue persists. It also wouldn't explain why the emails worked before, as I didn't changed anything in Windows Defender since then. As I wrote before, Thunderbird works fine with all other my email accounts except the Hostgator ones. Also with Authentication - normal or encrypted the issue persists. I contacted Hostgator support and they analysed everything, they did the steps with me to make sure I was inserting the correct info, and on their side everything is working normally.

I disabled Windows Defender and also tested with opening an exception in it for the profile folder, but the issue persists.

That was meant as a generic advice to make you aware there is an active anti-virus software running, and to avoid problems in the future. I didn't expect this to solve your problem at hand.

I have these 2 Hostgator email addresses that were working before and something happened that I stopped receiving and sending emails from those 2 accounts.

Was there any error message when the problem occurred first?

Is there anything account related in the error console (Ctrl-Shift-J) when you attempt to set up the account?

Did you try Manual Configuration when setting up the account? https://support.mozilla.org/kb/manual-account-configuration

Was there any error message when the problem occurred first?

- I just noticed Thunderbird was taking a long time to read the inbox of those account until it gave the message "Connection to server timed out".

Is there anything account related in the error console (Ctrl-Shift-J) when you attempt to set up the account?

- Yes, it says "Unable to log in at server. Probably wrong configuration, username or password." But I didn't change neither username neither the password on both email accounts.

Did you try Manual Configuration when setting up the account? https://support.mozilla.org/kb/manual-account-configuration </blockquote>

- Yes, I did which was also what Hostgator support recommended.

Thank you for your time.

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode

Does the problem go away?

christ1 said

Try to start Windows in safe mode with networking enabled. - Win10 https://support.microsoft.com/en-us/help/12376/windows-10-start-your-pc-in-safe-mode Does the problem go away?

No, issue persists.

have you purchases SSL certificates for the mail servers? or are you relying on some for another domain and need to ensure they are current and exceptions exist in the certificate manager?

Matt said

have you purchases SSL certificates for the mail servers? or are you relying on some for another domain and need to ensure they are current and exceptions exist in the certificate manager?

Hostgator offers free SSL for the domain. I contacted them again and they said the MX records needed to be updated. The guy did it and said I had to wait 1 to 8 hours for propagation. It's been more than that and the issue persists.

Try both of these sites to test one of the domains. Both very helpful in determining issues.

https://tools.dnsstuff.com/ and https://www.htbridge.com/ssl/