Comcast recommends port 587 with STARTTLS security, or port 465 with SSL/TLS security.

https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

When you changed the port, did you change the security?

When I changed the port in T-bird to 587, I left the security at SSL/TLS. When I configured Outlook, it defaulted to port 587 and security was also SSL/TLS, and that worked for Outlook but not T-bird.

But on your recommendation I set T-bird to 587 and STARTTLS. Sending an email with that resulted in

Sending of the message failed. An error occurred while sending mail: Outgoing server (SMTP) error. The server responded: resomta-a2p-647974.sys.comcast.net resomta-a2p-647974.sys.comcast.net ESMTP server not available. That is essentially the same msg as before, except the reported server name changed, from resomta-h2p-555031.sys.comcast.net to resomta-a2p-647974.sys.comcast.net. I understand that 'smtp.comcast.net' is just an alias that gets redirected to an actual server, so I'm guessing that changed name means the redirect is going to any of several redundant servers, and the fact that the exact name in a specific error message is probably immaterial to my problem.

I have set it back to 465 and SSL/TLS pending further advice.

To summarize and compare . . . We have 5 devices with which we regularly access our Comcast/Xfinity (as well as Gmail) email; 2 laptops, 2 Android phones, and 1 tablet. Here’s a description of each, and results of attempting to send email from each.

Device 1: Dell Inspiron 3583; Windows 11 Pro; Using my Comcast email account. Email client: Outlook SMTP server: smtp.comcast.net Authentication method: Normal password Port: 587 Security: SSL/TLS Test result: Success

Port: 465 Security: SSL/TLS Test result: Long delay in ‘Drafts’ folder, then ‘draft’ message was tagged as “Something went wrong and we haven’t sent your message yet”.

Email client: Thunderbird 140.1.0esr (32-bit) SMTP server: smtp.comcast.net Authentication method: Normal password Port: 465 Security: SSL/TLS Test result: failed; resomta-a2p-630472.sys.comcast.net ESMTP server not available.

Port: 587 (replicating the Outlook config.) Security: SSL/TLS Test result: The message could not be sent because the connection to Outgoing server (SMTP) smtp.comcast.net timed out.

Port: 587 Security: STARTTLS (recommended for port 587 Test result: resomta-h2p-555058.sys.comcast.net ESMTP server not available.

Device 2: Dell Inspiron 3511; Windows 11 Pro; Using my wife’s Comcast email account. Email client: Thunderbird 128.13.0esr (32-bit) SMTP server: smtp.comcast.net Authentication method: Normal password Port: 465 Security: SSL/TLS Test result: failed; resomta-c2p-555954.sys.comcast.net ESMTP server not available.

Device 3: Smart phone, Motorola Edge (2022); Android 14; Using my Comcast email account. Email client: Gmail ver. 2025.07.13.782672620 SMTP server: smtp.comcast.net Authentication method: Normal password Port: 465 Security: SSL/TLS Test result: failed, no further information; email left in Outbox. (note: when I delete the mail from the Outbox, it deletes, but app reports “Problem Syncing”.)

Device 4: Smart phone, Motorola Edge (2022); Android 14; Using my wife’s Comcast email account. Email client: Gmail ver. 2025.07.13.782672620 SMTP server: smtp.comcast.net Authentication method: Normal password Port: 465 Security: SSL/TLS Test result: failed, no further information; email left in Outbox. (note: when I delete the mail from the Outbox, it deletes, but app reports “Problem Syncing”.)

Device 5: Tablet; Android 14; Using my wife’s Comcast email account. Email client: Gmail ver. 2024.08.25.668103139 SMTP server: smtp.comcast.net Authentication method: Normal password Port: 465 Security: SSL/TLS Test result: failed, no further information; email left in Outbox. (note: when I delete the mail from the Outbox, it deletes, but app reports “Problem Syncing”.)

Outlook uses the windows certificate store for encryption certificates. Thunderbird uses the Mozilla one.

My guess is your antivirus has placed a copy of their self signed encryption certificate in the Windows store so Outlook works. It can not programmatically hack the chain of trust in the Mozilla store and you need to add it manually or disable the scanning that is causing the issue. Just a guess based on Outlook working and Thunderbird not.

As there is an extension to that error that refers to excessive failed authentication attempts you might also want to ensure that the server is set in SMTP settings to require a normal password. Most issuers result from folks that do not know their password changing that to none. It does not work but it does give them a new message to work on. No helpful, but new to them

It can not programmatically hack the chain of trust in the Mozilla store and you need to add it manually or disable the scanning that is causing the issue. Just a guess based on Outlook working and Thunderbird not.

How would I "add it manually or disable the scanning"? Is this something I'd have to do in my anti-virus (McAfee)?

Another bit of info I just now thought to check. I actually have two emails set up in T-Bird. In addition to the Comcast account that is giving the problem, I also have a Gmail account. It still works flawlessly. Comparing the SMTP settings, I see that the Gmail has "Authentication Method: OAuth2", while the Comcast is set for "Normal password". As a test, I changed that to OAuth2, but the result was the same, so I set it back to normal password.

One more clue ... Starting to poke around in McAfee for anything related to "encryption certifiates" I remembered that it also has a VPN, which I use when away from home. Without changing anything in T-Bird, I turned on the VPN, then tested again. This time sending the email works. I'm not sure what it reveals, but seems like an important clue.

Suddenly, the problem has simply disappeared!

Today, without changing anything - all smtp settings exactly where they've always been - I attempted another test email. This time, instead of the "server not available", it prompted for a password. I manually copied the pswd out of my password keeper (BitWarden), pasted it into the prompt, and everything went through just fine.

So now T-bird is working again, but as I described in an earlier message, I also had problems with the Gmail client on my Android phone - only on the Comcast account. Trying a test email there, the message went into the Outbox folder, with a status of 'pending'. Then I noticed there was a background alter, prompting for a password. Again, just like in T-bird on my laptop, I copied and pasted the password, and everything went through just fine. On both platforms, subsequent tests were seamless, no errors, no password prompts.

Based on all this, I'd say the problem was at Comcast's end. However: 1) they had denied there was any issue on their end 2) Using Outlook had no problems

I'd love to discuss this on a broader forum - one that's not focused on problems in a single product. I'm not sure where that would be. In 2021 I retired from a 40-year career in IT, and it was not uncommon to have to get experts from multiple areas (app developer, network admin, database admin, OS admin) to work out a problem.

I have the same problem: t-bird on windows 11 cannot send mail with Comcast. It trys but hangs forever (see screen shot).Clearly all of the above settings are incorrect. Where is the screw up? T-bird worked fine for me on Win 10 and Win 7.