Comcast controls your IP address. I suggest you contact them like the message says to do.

Thank you. I spent hours on Comcast's tech support chat. The problem apparently isn't theirs. I can send mail from my Comcast account direct from their server with no problem. But not via Thunderbird.

The messages is from their server. It tells you to contact them. How is this not their problem. Sending via webmail proves you know your username and password and nothing else. That does not use the same servers that an email client like Thunderbird uses and is issuing the error. They are either too stupid to know that or just used that as an excuse to get you to go away.

Here is the settings that Comcast recommends so do not guess. Use the correct ones https://www.xfinity.com/support/articles/email-client-programs-with-xfinity-email

Those are the settings I've been trying to use. Your information about Comcast servers is very helpful -- I didn't understand that. I'll return to them for more help. I really appreciate your taking the time to respond. Thank you.

You are not alone. https://support.mozilla.org/en-US/questions/1219571

I know the FBI is recommending everyone reboot their internet router due to some foreign hacking. I wonder if this could be related? I just rebooted my Comcast router but I have not had the blocked IP error yet.

Has anyone gotten Comcast to listen? I am getting same error - and while I can work with the browser software it is not as good as Thunderbird.

An error occurred while sending mail: The mail server sent an incorrect greeting: resomta-po-20v.sys.comcast.net resomta-po-20v.sys.comcast.net CSI IP 209.117.80.66 is not permitted to send messages. Please contact support if you feel this is in error..

Efultz, are you still experiencing this error? Have you tried rebooting your router? If you power down the router for about 30 seconds when you reboot the router it should reset the IP address. I am just curious if this will solve the issue.

I get this issue when using Thunderbird from work - and, I think, from home but now that I think about it I can't recall if I have tried it there. 99% of the time I work with my email during the day.

Guys, I really appreciate your help and ideas. I did reboot my router as the FBI suggested. Made me feel a bit safer, but did nothing to solve the Thunderbird problem. I don't want to keep spending time on this. I've had a gmail account for a good while, which I treated as secondary to Thunderbird. I've switched those around so that gmail is now my main email client. It handles my comcast addresses with no problem. Also, gmail has been seriously upgraded, with a much nicer user interface. It'll be my new happy home in the email world. What I do hate to give up: Thunderbird has a much more useful folder system than gmail's "labels." You can't have everything.

I am having the same problem which has been resolved a couple of times in the past couple of weeks after calling Comcast: https://support.mozilla.org/questions/1219571 - I just tried a test and I am getting the same error - no outgoing email is allowed due to a "greeting" problem. So this problem, for me, seems to be intermittent

I now believe that, for me, this is not a Thunderbird issue.

I contacted my VPN provider, Private Internet Access (PIA), and they said that they are now whitelisting SMTP servers. They want me to send them the IP address of Comcast's SMTP server. Since Comcast probably has more than one server (maybe hundreds), I suggested to them that they contact Comcast and work it out with them. I also pointed out that they could ping smtp.comcast.net to get the IP. I have not heard back.

PIA says that they have allowed outgoing email through Gmail, Yahoo, Outlook, and a couple of others because those service providers are using spam filters to reduce evil-doer activity. I assume that means that Comcast and Aplus are not using these filters.

My current solution is to disconnect the VPN in order to send email. This is not, of course, what I paid (PIA) for and I will deal with that.

I have not been able to find a solution to this problem with Comcast. I have to go to the web interface if I want to send email - Thunderbird works for all else. Only saving grace is that if I click on the sent folder in Thunderbird it downloads the sent emails from Comcast so I am able to continue using Thunderbird as my primary repository. I just wish Comcast would get their house in order.