THe problem is not Thunderbird, it's your settings. Thunderbird is just a messenger. Maybe if you post your complete SMTP settings, someone here can advise of your problem.

Thanks for your reply but I had my ISP look at the settings extensively and found nothing amiss. I have run Windows and Malwarebytes scans that found nothing. SPAMHAUS has been particularly unhelpful in identifying the problem and they continue to block Thunderbird message sending. SPAMHAUS suggested my router may be infected but I just disconnected from D-Link support and was told their routers can't be scanned for malware.

I can receive email and when using my ISP's webmail I can send messages. Here is some of my configuration specs:

SSID: Dorval-5GHz Protocol: Wi-Fi 5 (802.11ac) Security type: WPA2-Personal Network band: 5 GHz Network channel: 36 Link speed (Receive/Transmit): 117/117 (Mbps) Link-local IPv6 address: fe80::ed7a:4561:b3c6:20b%9 IPv4 address: 192.168.1.174 IPv4 DNS servers: 8.8.8.8 8.8.4.4 Manufacturer: Intel Corporation Description: Intel(R) Dual Band Wireless-AC 3168 Driver version: 19.51.30.1 Physical address (MAC): D4-25-8B-E4-4C-AB

More internet settings. I have tried both of settings but remain blocked by SPAMHAUS. Very frustrating and have spent most of today trying to fix this.

Funny how these things keep coming up. And somehow the ISP that misconfigured their SMTP server filtering always wants to blame the mail client that they are rejecting. So the error is for a server with the IP address given as 208.181.95.35

Spamhaus specifically says this is a server an the following is why thge IP is listed.

The most recent connection was on: December 1 2023, 13:55:00 UTC (+/- 5 minutes). The observed HELO  value(s) were:

208.181.95.35 2023-12-01 13:55:00 ip-109-42-178-239.web.vodafone.de
208.181.95.35 2023-01-29 17:20:00 232.191.95.93.triolan.net

A check on the actual IP address indicates it is actually a Google mail server.

181.208.85.209.in-addr.arpa. 86400 IN PTR mail-lj1-f181.google.com. 

So I think I just dropped into a time warp back to about 2012 when Shaw somehow managed to be using the same IP addresses for their mail servers as Microsoft used for HOTMAIL and they blamed Thunderbird for rejecting the Hotmail encryption certificate as not valid for a SHAW.CA server name.

Based on my assumption that the issue is Shaw, and probably leased capacity and a gut feeling it might well be DNS that is at issue here, 8.8.8.8 is Googles DNS server, not shaws. I suggest you dig into Thunderbirds settings page and enable the use of DNS over HTTPS. This will change the DNS resolver to cloudflare and just might fix the problem which has me confused by the very circular nature of the name resolutions.

It might help to use the correct server settings: mail.shaw.ca on 587, STARTTLS security, authentication = normal password, User Name = part of address before @.

Thanks for your reply. I wasted may hours in frustration trying to fix this but this morning it seems the SPAMHAUS blockage has expired as I can now sent emails using Thunderbird. I do suspect the problem was caused by my ISP, Shaw. I ran three malware scans, the last took 15 hours, and nothing was found on my devices. A VERY FRUSTRATING experience. SPAMHAUS was very useless in solving and they never notified me of the removal. The ISP support help was almost as useless.

Bottom line, I am happy to continue using Thunderbird and will investigate signing up with a different ISP.