What are your settings? Earthlink offers secure and non-secure options:

https://support.earthlink.net/articles/email/imap-email.php

What's the antivirus, and its email/SSL scanning settings? What error message do you see?

IMAP security settings - Connection Security = None, Authentication Method=Password, transmitted insecurely SMTP security settings - Connection Security = None, Authentication Method=No authentication PC Matic Email Antivirus - don't know the email settings IMAP error message = Failed to connect to server imap.earthlink.net SMTP error message = Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) smtpauth.earthlink.net is unknown. The server may be incorrectly configured. Please verify that your Outgoing server (SMTP) settings are correct and try again. Thanks

Which ports?

IMAP = port 143 SMTP = port 587

Thanks

If you are going to use connection security = none for the incoming and outgoing servers, the authentication for both should be 'password, transmitted insecurely', according to their instructions.

If you choose STARTTLS security for incoming and outgoing, the authentication should be Encrypted password.

Changed both settings to 'password, transmitted insecurely' and tested. Same error messages.

Thanks

Does Earthlink ISP need to upgrade to NIST SP 800-52 Rev. 2 for TB 78.4 to function properly, in other words TB 78.4 requires that TLS 1.2 be implemented?

If Earthlink's mail servers don't support TLS 1.2, you can change a preference in TB 78 to make it work with their servers:

https://support.mozilla.org/en-US/questions/1295861

but if the connection security is 'none', the TLS version shouldn't matter.

I noticed from another topic that if you have STARTTLS security, the authentication should be 'Encrypted password' for both servers:

https://support.mozilla.org/en-US/questions/1294103

I am having very similar problems with 3 Earthlink IMAP accounts, although some details vary between accounts. All the accounts are Server = imap.earthlink.net, Port = 143, Outgoing = smtpauth@earthlink.net, Security = STARTTLS & Encrypted Password

See my full description at https://support.mozilla.org/en-US/questions/1310338

Hello I spent hours over hours with the same issue which appeared directly after updating to 78.4. My web asccount is on 1&1 Mail & Media side, via mobile or web access no disturbance. Settings are unchanged compared to previous ones. What has been changed for that version?

WITH SO MANY COMPLAINTS ABOUT THE NEW VERSION 78.4 WHEN IS IT GOING TO BE FIXED??? THIS HAS TO BE A PROBLEM WITH THUNDERBIRD.

WITH SO MANY COMPLAINTS ABOUT THE NEW VERSION 78.4 WHEN IS IT GOING TO BE FIXED??? THIS HAS TO BE A PROBLEM WITH THUNDERBIRD.

Hijacking and spamming other people's threads won't get you help any faster. Also, all caps text is an eyesore and it implies you're shouting, which is just rude. State in your original thread what you need help with.

Yes - that is my view too So I installed the latest stable version 68.9.0 - as expected no further issues. To avoid same issue I ticked the option to be asked for installation when a new version is avaialble. But this choice was not considered on reason I do not know - so back to 78.4 with the same error, also 78.4.1 does not solve it. A solution is really needed...

Going on 3 weeks now. Have tried every fix posted short of uninstalling TB 78.4. I refuse to do that now. Don't want to loose any data and profiles. It happened before. Still cannot get TB 78.4 to function properly with my primary email ISP. I am accessing my primary email via ISP web based email and also using TB 78.4 by forwarding email from my primary email address to another secondary email address/ISP that will function correctly in TB 78.4. Wow.. very inefficient and a pain. In response to comments from Stan regarding Duane's complaints in CAPS; duh.... your probably right Stan. Duane probably is emphasizing his complaint with CAPS. I understand his frustration. I understand this is free software and certainly appreciate the work that all have done, especially the contributors to these posts. I have not seen any official response from the TB team as to when or if this will all be corrected. Just "band-aid fixes" from these posts that have corrected some of the users problems and not mine or others. No version update fixes. I have been using TB for many years without any real issues and it has served me well, however the TB team needs to at least respond with some kind of official communication indicating a version fix or a non support statement. I would welcome and thank any help beyond what has been posted. Again, thank you.

Just checked 11/06/20 TB 78.4.1 Release Notes.... no listing under Fixes for the problems indicated in this chain of posts and no listing under Known Issues, Unresolved for problems indicated in this chain of posts. It appears that TB team does not know they have this problem or they are ignoring or not responding. It (this problem) certainly doesn't appear on the Known Issues Unresolved. What is going on here? Very interesting Customer Support!

Checked 11/09/20 TB 78.4.2 Release Notes......no listing for the problems indicated in this chain of posts. TB team still has not acknowledged any of the problems listed here. Please communicate with your customers and at least acknowledge a problem or non support statement, so I can decide if I need to move on to another email client. Hello...... anybody home? Thank You

I have this problem with my gmail and comcast accounts. Both stopped downloading yesterday after I installed the update. I expect more from Mozilla.

TB 78.4.3 released the evening of 11/11/20 and mysteriously (there were no references to this problem in the release notes) all my incoming email downloaded (it took awhile since I had no downloads from 10/22/20 to 11/11/20) to my primary email account with Earthlink ISP. Incoming email continues to downloaded correctly, other than an occasional error message that clears itself after a short period of time (??).

However, I still can't send any email from the Earthlink account. I am receiving this error message: Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) smtpauth.earthlink.net is unknown. The server may be incorrectly configured. Please verify that your Outgoing server (SMTP) settings are correct and try again.

Once again, I tried all the different "band aid" fixes with no success. Maybe the next TB release will mysteriously correct the outbound send problem. Again, if anyone has any new fixes, the help is appreciated. Thanks.

edited for formatting

Well it is now a month since I been unable to use TB with my Earthlink provider and my primary email......... the ongoing saga. My Earthlink account is now back to where I started from. Inbound email downloads just mysteriously stopped like they started. No inbound emails, no outbound send capabilities. TB team is a total failure. No acknowledgment of any problem. No communication. No nothing. Did you guys die from Covid? Since you left Mozilla you have gone downhill and TB is now totally unsupported junk software. I am now searching for a replacement email client. Wow..... what a complete lack of any kind of support.