We replaced our gateway panoramic modem a couple of weeks ago and my brother has since been having trouble getting Yahoo emails using Thunderbird. He had no trouble with it before we replaced the mo...
I think when OP said modem they meant Panoramic gateway, which DOES have a firewall and NAT. I haven't heard of either interfering with email clients, but you never know. If they wanted to rule all that out and put the gateway in "water hose" mode, they could enable bridge mode. That's assuming the PC having issues can connect via ethernet. I think other things like you suggested should be checked first.
They said their GMAIL was working, and it uses the same ports. If a mail client is working with one mail service, then there may be something different in the configuration of the other, hence I suggested verifying the ports?
"I think when OP said modem they meant Panoramic gateway, which DOES have a firewall and NAT."
Yes, it is a Panoramic WiFi modem, CGM414X, and the previous model was CGM4141.
"That's assuming the PC having issues can connect via ethernet."
It was tried on a desktop PC running Linux and on a laptop running Windows 10, both using WiFi. The desktop is not close enough to try ethernet but the laptop could.
The firewall was set to minimal while this was happening, and I just tried the custom setting of disabled and it is the same. All of Thunderbird, Claws Mail, and Vivaldi browser's builtin client were previously working fine and the settings had not been changed: IMAP port 993, SMTP port 465, using SSL. All of the Yahoo, GMail and Vivaldi mail accounts use the same ports and SSL, but only Yahoo has the problem, and only on this WiFi network. The error is always a timeout connecting to imap.mail.yahoo.com
Edit: Just tried with the laptop plugged into the modem via ethernet, same thing timing out connecting to Yahoo while GMail still works.
With a new modem, you get a different IP address. Yahoo may actually be blocking your new address, go to www.whatismyip.com. get your new WAN IP, and contact Yahoo and ensure they don't have a block on it.
Also, you can do a ping to imap.mail.yahoo.com as well as a tracert imap.mail.yahoo.com
I verified both respond to ping and tracert, and was able to tun both without issue. The tracert should give you an idea where the failure is,
5 16 ms 14 ms 14 ms ashbbprj02-ae2.0.rd.as.cox.net [220.127.116.11] 6 15 ms 14 ms 12 ms 18.104.22.168 7 20 ms 20 ms 20 ms ae-0.pat2.nyc.yahoo.com [22.214.171.124] 8 34 ms 34 ms 33 ms ae-1.pat2.bfw.yahoo.com [126.96.36.199] 9 29 ms 30 ms 29 ms unknown.yahoo.com [188.8.131.52] 10 49 ms 57 ms 40 ms et-11-1-1.clr2-a-gdc.bf1.yahoo.com [184.108.40.206] 11 33 ms 32 ms 31 ms lo0.fab3-2-gdc.bf1.yahoo.com [220.127.116.11] 12 29 ms 31 ms 28 ms lo0.usw2-1-lbc.bf1.yahoo.com [18.104.22.168] 13 30 ms 32 ms 31 ms jimap400-3.mail.vip.bf1.yahoo.com [22.214.171.124]
Thanks, Darkatt. I recall that around the beginning of the year the WAN IP had changed, but none of our software or devices was affected. I've already tried some pings but not a tracert. Pinging www.yahoo.com was successful but pinging imap.mail.yahoo.com returned the message "Destination host unreachable"
I will check later what the new WAN IP is and compare a tracert with yours