Are you using a VPN? If so, try disabling it.

I had not changed any settings before this issue started. I have been using a VPN with no problems, and I really would rather not disable it when working wirelessly. However, I had missed the response to tommymeisel regarding the same problem (trouble sending with Comcast); it suggested disabling the antivirus scan of outgoing mail. That seems to have resolved it for me. Appreciate any and all suggestions in any case!

Well disabling virus scans on outgoing mail did work--once, but no more. But I was able to send a message when I paused my VPN. This is not an issue with my ISP. I need to request that Mozilla fix whatever is preventing messages from being sent with a VPN active. I don't think it should be necessary to give up my VPN connection when I'm on wifi.

Well disabling virus scans on outgoing mail did work--once, but no more. But I was able to send a message when I paused my VPN. This is not an issue with my ISP. I need to request that Mozilla fix whatever is preventing messages from being sent with a VPN active. I don't think it should be necessary to give up my VPN connection when I'm on wifi.

It is not a TB issue. Your email provider is blocking your sent messages because of the VPN.

Here is the proof:

"Outgoing server (SMTP) error. The server responded: resomta-h2p-555359.sys.comcast.net resomta-h2p-555359.sys.comcast.net ESMTP server not available (62240/216.183.125.238)." That error message is coming from you provider, not TB.

"My provider says the issue is not at their end because my server settings are correct." It doesn't matter. The VPN changes the IP you're sending from, and the provider is blocking that IP.

Thanks for that info, Ed. Because of that very language, I went to my provider before I came to Mozilla with this. And I have no issues logging on to my ISP and sending or receiving mail via their webmail function. But I'm learning that my problem does appear to be VPN-related. That is, when I'm using TB, there's trouble with some VPN servers and not with others. I've observed no problems switching VPN servers for use with webmail. If I have to pick and choose which VPN servers I use with TB, then so be it, I guess. Onward....

webmail has an entirely different mechanism for getting and presenting messages, so it's absolutely expected that vpn wouldn't interfere.