Thunderbird Community Forum

I keep getting emails like the one below. How can I stop Proofpoint from blocking emails like this? I can't get any response from Proofpoint.

Thank you.

Subject: Mail delivery failed: returning message to sender Date: Mon, 23 Mar 2026 15:32:49 -0400 From: Mail Delivery System <Mailer-Daemon@server240.web-hosting.com> To: [edited]@unexpectedwildliferefuge.org

This message was created automatically by mail delivery software.

A message that you sent could not be delivered to one or more of its recipients. This is a permanent error. The following address(es) failed:

[edited]@marshallsterling.com host mxb-00261401.gslb.pphosted.com [148.163.148.189] SMTP error from remote mail server after initial connection: 554 Blocked - see https://ipcheck.proofpoint.com/?ip=66.29.141.68

I’ve tried all versions of Thunderbird on my 2020 M1 Mac, and even though I log in correctly, it doesn’t load emails from my account. I used to use Thunderbird normally because I needed it, but I haven’t used it for two months, and now I see that it doesn’t work when I try to connect to my email. It connects to my email normally, but the problem is that when I click on “Inbox,” there's nothing there, and when I click “Fetch Mail” to try loading it again, a message pops up at the bottom saying everything is fine, but in the end, I still don't have those emails. What kind of error is this?

I have been using Thunderbird with two POP3 Hotmail accounts for some time and I am trying to set up a third account with exactly the same configuration. However, both the automatic and the manual configurations failed. The new account receives mail, but the outgoing smtp server config seems to have a problem. Sending messages times out or keeps prompting for a new password.

The Hotmail account has POP3 and IMAP enabled. Thunderbird is set to receive all cookies. I cleared history and cache after every attempt and also temporarily disabled the firewall.

Any suggestions about what the problem could be?

Thunderbird version number: 148.0.1 Mail provider: Yahoo Client OS: Windows 10

The issue and steps I tried to resolve: On 1 Jan 2026 I switched from Outlook client on my Windows 10 PC to Thunderbird 146.0.1. I did not import any user data from Outlook but started with a clean install of TB for the new year. I use yahoo.com with POP3 as my email provider. Server Settings: Server Name "pop.mail.yahoo.com", port 995, SSL/TLS, OAuth2 Outgoing Server: Server Name "smtp.mail.yahoo.com", port 465, SSL/TLS, OAuth2

it all worked fine and I was very happy with the setup. Then from early to mid-February I was unable to use my computer. When I finally accessed Thunderbird again, it connected to Yahoo and showed that approximately 800 email messages would be downloaded and started downloading them. After downloading about 100 messages, Thunderbird timed out and I haven't been able to get it to work since.

I have read and followed or tried to follow these articles plus others that I may have forgotten about (not listed in any particular order) - OAuth2 Authentication for Yahoo, AOL and ATT | Thunderbird Help - including all the steps in Please check the following before asking for support - POP access settings and instructions for Yahoo Mail | Yahoo Help - Automatic Account Configuration | Thunderbird Help - Manual Account Configuration | Thunderbird Help - Thunderbird and Yahoo | Thunderbird Help - Send and receive messages in Thunderbird | Thunderbird Help - Can no longer get emails | Thunderbird Support Forum | Mozilla Support - Emails suddenly stopped downloading to Thunderbird 140.2.1esr | Thunderbird Support Forum - Get Messages | Thunderbird Support Forum | Mozilla Support - Cannot receive messages | Thunderbird Help - Problem with Thunderbird working with Yahoo Mail acccounts. | Thunderbird Support Forum

I have removed all messages from the Yahoo inbox and tried to find one or more messages that may be affecting the download. I don't know the best way to determine that.

I use Windows Defender for my antivirus. I don't see anything in Defender that blocks or scans email messages which would prevent downloads, although I could be wrong about that. if this might be the issue, please provide some guidance about how to check Defender.

I have uninstalled and re-installed Thunderbird. In the process I downloaded the setup file for Thunderbird Setup 148.0 and installed that version. It later auto-updated to 148.0.1. Before installation, I removed all external connections from my Yahoo account and completely removed Thunderbird from my PC, including the profile and profile.ini. When I reinstall and switch the account to POP3, Thunderbird captures the App password from Yahoo and connects just fine. But it still times out while attempting the first download of a message. The first several times it timed out, an error message was displayed but now it doesn't show anything. Except no messages are downloaded.

At this point, I don't know what else to try. Since I don't have much experience with Thunderbird, I don't know how to troubleshoot the process (logs to look at, etc.). I don't want to go back to Outlook but I need the email client to work and download messages. Any help would be much appreciated.

Using TB 140.8.1 esr(64 bit) on Win11 Home desktop. My internet speed: About 40 Mbps up and down. All routine Win and TB updates installed when available. No changes in configuration, accounts, settings etc. except installing routine updates to TB and Win11. No problem with sending/receiving email. Sent mails go out immediately and appear right away in my sent folder. No problem receiving USENET news new postings to subscribed groups. I'm subscribed to 3 groups, using news.eternal-september.org. It used to be that when I wrote a reply to a post in a thread the msg would be sent within a few seconds. Recently and consistently, when I post to a group, pop-up notification tells me msg is being assembled. But progress appears to stall and msg often takes an hr. or more to appear in sent folder. Sometimes, it never appears in my sent folder but I can see it was sent because subsequent new postings to the same thread often cite my text and/or reply to it. My sent configuration: outgoing server: smtp.gmail.com port: 465 user name: my gmail email address - which works fine using google's web interface or TB authentication: OAuth2 connection security: SSL/TLS Any idea what's wrong? Any suggestions what to try to get back to prompt processing of sent postings to subscribed USENET groups?