It may help if you post screenshots for incoming server settings and the SMTP server settings.

I've captured screenshots of the various server settings on both machines, but is there a privacy issue if I post them here? I wasn't sure.

True. You could block out email address or post the text from the the settings, e.g., server, port, authentication, security. be sure that the settings have your complete email address, although not needed here.

OK, here are some screenshots I captured into a document.

Roadrunner setups are all over the place, so I cannot be positive, but I suggest you change your incoming port to 995, SSLTLS security. for outbound, I suggest port 587 and also SSLTLS or STARTTLS (if one doesn't work try the other). let us know how it goes. Your current settings are insecure and many email hosts will reject it.

When you changed the IMAP setup to POP, did you add the account as POP, or just change the settings? The latter won't work:

http://kb.mozillazine.org/Convert_a_POP_account_to_a_IMAP_account

Spectrum recommends these settings:

incoming IMAP: mail.twc.com, 995, SSL/TLS, normal password, User Name = email address

outgoing SMTP: mail.twc.com, 587, STARTTLS, normal password, User Name = email address

https://www.spectrum.net/support/internet/mobile-email-setup

Well, now I'm even more confused! Now I can't set up a POP3/SMTP account with the same account name as the IMAP one that was automatically installed -- not too surprising, but I didn't get into this hole before. When I try to create a new account, it appends _2 to the name, which didn't happen before. I hesitate to delete the IMAP account, though I could start from a fresh TB installation, set it up as you suggest, and import from my profile on the old machine. (BTW, I was disappointed that the message filters were lost in the Export/Import process.) I also have no idea why the IMAP account won't send messages. Should I just start over?

Also, my previous PC still gets and sends messages just fine with what I showed in the 'old machine' screenshots, and both of the PCs are running Windows 10 and the current version of TB. I'm currently doing all my email there, but that PC is getting flaky -- sometimes it takes literally minutes to display a new webpage, and I don't know how long it will last, which is why the replacement!

reply to sfhowes; Well, I tried to avoid the complications of doing the transfer by using a local service (Geek Squad, affiliated with Best Buy), and he apparently installed a TB that defaulted to IMAP. I was trying to convert from that to POP3/SMTP, and got into all this. If you look at my screenshots, you'll see that I did try to add the account as POP. The article that you cited was for the POP-to-IMAP conversion, don't know how similar the other direction is.

Thanks for the inclusion of the recommended outbound SMTP server, I'll have to try following up on that -- didn't know the same account could have both. More later....

OK, using the configuration in sfhowes (the only thing I needed to change was the ports), I sent myself a test message. I got a warning about it not being saved in the Sent folder, but the message got through, so I think I'm OK now! Thanks so much for your help, both david and sfhowes. I really appreciate it. I'm checking the Solved on the sfhowes message now, since that's the configuration I used and it apparently succeeded. Presumably saving my profile on my backup device will still capture all the messages, so I really shouldn't need POP. Now to set up my message filters, and I should be back to normal operations.