Have you asked your mail provider why your IP address is being blacklisted. Because you are waiting for them to fix the issue, it has nothing to do with Thunderbird at all.

Common reasons include sending unsolicited mail that has your IP address listed with one of the reputation services as a spammer, another oldie but a goodie is sharing a server in a hosted situation with a spammer. Colloquially called being in a bad neighborhood. They get blacklisted and because you share an IP address you go along for the ride.

Thanks for your reply, Matt, but I'm not sure the mail provider is the problem -- altho I'll have to contact them and ask.

The reason I'm not sure is that, altho Thunderbird will not download my email, I can go to my provider's site [Comcast] and access my email via Comcast's webmail. One would think that, if they were the one's blocking me, my webmail would be blocked also.

However, I admit that I don't know enough to know if this is sound reasoning.

Comcast is a mess. To many mergers and not enough effort put into the consolidation. Add that to a bad server software solution and you will start to get a picture. Then they employ "support" staff that read scripts to customers. It is unusual for them to have any real skills in computing or email specifically.

The most important thing with regard to comcast is their SPAM tools affect their own servers and they do not use the same severs to send or receive mail on the web mail platform as they do for IMAP POP and SMTP. They are about as similar as fedex and the postal service. Both convey packages but there the similarities end.

Comcast has been a perennial issue where SPAM tools are concerned. At one time their SMTP server refused to accept emails from home user IP addresses because they were listed as home users IP addresses and as such were no allowed to send mail to Comcast.

They also used to require all mail send to go out through their severs, not they are being called spammers because their customers are sending all manner of email addresses via Comcast SMTP. Make sire you use Comcast outgoing servers for @comcast addresses. Anything else needs to have it's own outgoing server setup in Thunderbird with the appropriate server. Sending from your hosted domain email or Gmail for example should not be done via Comcast.

Thanks, Matt. Yes, Comcast has more than its share of troubles, but, for where I live it's the best -- almost the only -- choice available.

However, I did take your advice and contact Comcast. They could find no problem with my address, but, while talking to them, it occurred to me that I use a VPN -- obtained thru my antivirus program, Avast. I turned off the VPN, and my email immediately downloaded in Thunderbird.

It's still strange, because I've had the VPN for maybe six months, so I don't know why it's started to occasionally interfere with my email. However, I guess I'll have to contact Avast and see if they have any suggestions.

Thanks again! Rodger

I think you need to ask yourself exactly what security a VPN providers for you, other than funneling ALL of your internet traffic through a server controlled and operated by Avast.

https://en.wikipedia.org/wiki/Avast

So who actually has access to your data? Avast I am sure are honorable company headquartered in Prague in the Czech Republic, but what countries does your data pass through in it's VPN quest, and which think they have a legal right to examine said data. These are questions everyone should be asking. Just like where is your cloud data stored. Here in Australia we find most of ours is in Singapore and subject to Singapore law.

Likewise this article https://thebestvpn.com/reviews/avast-secureline/ suggests hat Avast may be linking at least some of the data to Facebook and Twitter. They are offering you a service that is ostensibly free, so they are making money off you somewhere else, or they would go out of business.