So are you replying as say bill@comcast.net using the cable one SMTP server? That would probably be a technical violation of cable ones TOS and quite probably result in their spam system blocking you.

It used to be fairly common to do things like that in the USA, not for security reasons, so the ISP/Hosting provider could monitor your interaction and sell the data on to marketers. Finally they have mostly had to stop doing such things as spammers rarely use their own systems or mail addresses to send mail. So if you connect with say cableone, they will probably only accept mail from email addresses that end in @cableone.com or .net.

Looking at their web site, it looks like the email may be provided by Google. They accept email for non hosted domains, but only so much without going through a process of registering the address as one you own.

At the end of this, the issue is not with Thunderbird. The rejection is coming from your providers mail server. They have logs and all sorts of things you and I have no access to to answer questions.

So perhaps they can answer why their server is rejecting your mail. They are doing the rejection, so it is not really incumbent on anyone else but them to guess why they are doing what they are doing.

Matt, I'm not sure I understand, so let me supply some details. I used to have Cox Communication as an ISP in the Tulsa, OK area, and used their email for many years. Over 4 years ago I moved to a location about 30-40 miles outside their service area, and had to go with a new ISP - that is Cable One. I wanted to retain my e-mail address, and so kept email only service with Cox, but was forced to use Cable One to send email (Cable One's SMTP server) using Cable One's system. So I am sending e-mail through Cable One's SMTP server that DOES have a return address of XXX@cox.net. I get the error whether composing a new email or replying to one.

This setup of receiving mail from Cox and sending it out thought Cable One has worked just fine until 3 days ago. The Cable One support reps I talked consulted with top levels of their support when I was on the calls with them - calls that spanned several days. That top level of Cable One support is who determined it had to be a Thunderbird error.

So If I'm getting your drift, Cable One knows their server is the problem, but are lying to me about it. They are well aware I'm receiving mail through Cox. They have made it plain they can't help me, that everything is working on their end, and the problem is elsewhere (in Thunderbird). If that's the case, that leaves me up the proverbial creek without a paddle.

So Matt, am I understanding you correctly?

You do understand everything I say is a guess, I do not work for cable one, so I do not have access to logs.

The brain dead people over at yahoo decided at one time that email that had a reply to address were SPAM. So check your account settings in Thunderbird that you have not set that. There are still mentally challenged mail administrators out there. Yahoo doe not have a monopoly on them.

Do a test. Send a cableone mail (I assume you have an email account with them) and send mail from it with a cable one from ( you can edit it in the composer). Send it to your Cox account. Did it go through or bounce? If it went through then the problem is at cable one. They are not accepting your cox mail. simple as that. I don't care what their support said. If they refuse to accept the mail it is up to them to explain why they refused. It is not up to you to go cap in hand all over the place with vague determinations that is is not them.

One common thing I see from the truly smart ISP's is they blacklist all their own IP addresses from sending mail on their SMTP server. They "forget" they have users using mail clients. They are basically enforcing use of webmail.

Check your IP address at spamhaus. https://www.spamhaus.org/lookup/

Most blacklist/Blocklists are third party and they are one of the biggest. Check to see if your IP is blacklisted.

Thanks Matt! I took your advise and sent an email from my Cable One webmail to my Cox account, and it arrived OK. I also checked my public IP address at Spamhaus: it is NOT blocked.

So.... I'm going to have to migrate to a new e-mail address - a royal pain that, involving close to 100 critical institutions.

The course of least resistance would be to just setup my incoming mail on Cable One in Thunderbird, in spite of the fact that I hate getting into bed with anyone who screwed me. But I trust Google and Yahoo a lot less.

But your comment on "smart ISPs" essentially enforcing the use of webmail gives me pause.

So should I go to webmail? If so, which one would you choose? Pop3 email on Thunderbird has served me very well over the years, allowing me to do things like setup custom filters to sort incoming email.

Your advice and insights are greatly appreciated!

The path of least resistance is to set up your cable one account, leave your cox account there and simply reply when you need to using the cableone account. (Change the email address in the account settings for cox to cable one. That is in the entry above server settings for the account.

You could even use the signature switch add on to add a note to every email you reply to from the cox account notifying of your changes of email address if you wanted. If it is not subscription bases email, and if you are replying it rarely is, folks will eventually get the message.

You other choice that I know works is to use a Gmail account to send your mail. You register your cox account in their settings on the web site and click on the email they send to confirm you can actually access the mail account and they will happily allow what you have been doing to now with cableone.

Looks like our premise of the CableOne server rejecting emails because they contain a return address of XXX@Cox.net isn't all the problem. I set up Thunderbird to receive email from my XXX@cableone.net account, and attempted to send an email with that as the return address with the same SMTP server settings after double checking the settings and password: it failed. But the error message is not the same - see attached image.

I'm going to proceed with your recommendation to use my Gmail account as an SMTP server to send e-mail. I'd heard that was possible, but wasn't sure it was for real before your confirmation. I'm researching just how to do that now, and have found several articles describing the process. I'm fed up with CableOne's email "customer no-service"; they just may have done what "smart ISPs" have done in your comment above, but won't own up to it.

Thanks for your help!

I am not surprise that mail.cableone.MET is unknown. You have a typo.

You're correct of course - ya stare at this stuff long enough you start missing the obvious stuff. Corrected that, But in the process generated a new problem. I cleared all the saved passwords for outgoing mail servers, the re-entered the outgoing server settings manually. I expected Thunderbird to ask me for a password after doing this, but it does not. Nor can I find a place to manually input the passwords. So sending attempts fail with with both Gmail and CableOne servers as no password is going out during the log on attempt.

Wondering if I need to uninstall Thunderbird and re-install it to fix this, and if I do, do I dare import the old profile.

What anti virus? Norton and a few others have password "vaults" from which they happily dispense incorrect passwords on your behalf.

Did you remove all passwords, or only the ones you recognized. Your google password for instance will start with oAuth: in the Site. oAuth passwords have user names that look like Microsoft software license numbers. (GUIDs) Not anything you are familiar with.

My antivirus package is Webroot Internet Security Plus V9.0.27.49. It does contain a password manager, a feature added in my last upgrade, but it must be set up to be active. I've never set it up, and clicking on that feature in my Webroot console takes me to a Webroot account login page. After logging in, it takes to to a page that that indicates the Password Manager feature is not active, and needs to be set up.

When I removed the passwords from my Thunderbird account setup, I left the three in place for my three incoming mail accounts (two from Cox.net, and one from CableOne.net), and only those three. Those three incoming email accounts are working just fine. I removed all the saved passwords for the outgoing (SMTP) mail servers, and those were easy to identify. The Google password never got saved as it was never entered, so it did not show up in the Saved Passwords list.

I considered removing the saved passwords for those three incoming email accounts, but after removing saved passwords for the outgoing accounts, and not getting prompted for a password on send attempts, I was afraid the same thing would happen if I removed those.

I think post a screen shot of your SMTP settings... lets see if I can see anything you can't.

Matt said

I think post a screen shot of your SMTP settings... lets see if I can see anything you can't.

Here's the screenshot of my two SMTP server entries. I combined the settings from the 2nd account in the photo. I also included a screenshot of the Saved Passwords - note that neither of the SMTP servers appear in the list. The three that show are incoming email accounts, and those are working fine. When attempting to send from either SMTP server, I now get an hour glass, no error messages, and Thunderbird does not ask for a password, which it should since it has neither of the two are saved.

I'm getting by for the moment by logging into a CableOne's webmail to respond to an email, or to send a new email.

Unless you come up with an idea, my plans are to backup the Tbird profile, uninstall and re-install Thunderbird, and fill out all the server setting manually. It will be a real pain to restore my main incoming profile, as I have 31 sub folders to my main Inbox that I'll have to manually copy to the new profile, with filters to sort them into those sub folders. Then selectively attempt to import filters and address book using the "import" commands from the Tools menu.

Create a new profile, add one of the accounts and see if it works. That way we might get a clue as to it being in the profile or in the computer.

My blog post https://thunderbirdtweaks.blogspot.com/2019/09/i-lost-my-profilemail-on-update-to.html

The post has other ambitions, but it does explain how to access the about:profiles and how to create a new profile.

The other thing I was thinking might be worth checking is the error console (ctrl+Shift+J)

Once it is opened clear it with the trash icon, it is truly full of rubbish. Once cleared then click to send an email and note what appears in the error console. I am thinking particularly about certificates and encryption stuff. but any error that repeatedly appears when you click send is worth a look. (Right clicking errors offer a copy menu item.) I say repeatedly, as other background tasks like syncing calendars might cause an error to pop up once or twice as a coincidence, but if it comes up repeatedly it might be relevant.