[Users] can send but not reply from claws in comcast.net account

[George Avrunin]

Claws-mail 4.2.0 on Fedora 40 (from the claws-mail-4.2.0-3.fc40.x86_64
rpm on Fedora; claws-mail has not been updated recently).  For the last two
days, I have been strange having problems with my comcast.net account.

Yesterday evening, I noticed that claws-mail did not seem to be receiving any
messages on that account.  I logged into the account through the web interface
at xfinity.com and there were many messages, and a request from Xfinity (which
is a major US cable TV company and ISP, formerly called Comcast) to update my
password.  So I changed the password.  Updating to the new password (and
possibly restarting claws-mail) got all the messages.  But I couldn't reply to
a message from a frequent correspondent.  

Today, doing more exploration, I find that I can send a new email from the
comcast.net account without a problem.  The progress window comes up showing
it's connecting, then authenticating, and then the message sends successfully.
But if I reply to a message that has been received in the comcast.net account
(even a message I just sent from another account), the progress window shows,
connecting, authenticating, and then I get 'An error happened during the SMTP
session. You can try to "Send" again or queue the message with "Send Later".'
The network log just shows 
   ** error occurred on authentication
   *** Authentication failed:
   535 5.7.0 ...authentication rejected (6)
This doesn't seem to depend on the address I'm replying to, but I haven't
tried very many addresses.  (But some .edu ones and gmail.com ones.)

I am able to reply from this account using Thunderbird with no problems, so it
doesn't seem to be the case that my comcast.net account doesn't handle replies.
The Thunderbird account is using "SSL/TLS" and port 465 for sending to the SMTP
server (smtp.comcast.net).  In claws, I am using port 587 and STARTTLS, which
is what I've been using on this account for a long time.  If I use port 465
(whether I have claws set to use "TLS" or "STARTTLS" for smtp), I still can't
reply and I can't save the message to the Drafts folder when I can't send it.
In fact, I just tried this and I can't save to Drafts even after changing back
to port 587 and STARTTLS, though I saved this message successfully to Drafts
in order to check the port stuff again. ???

The fact that I can originate a message from the account suggests to me that
the configuration in claws-mail is correct (e.g., username, password, TLS
stuff, ports, etc.), but perhaps there's something different happening on
replies that I don't know about. These problems seem to have started
yesterday.  

I've looked through the dnf history for the last couple of days to see
what else got updated.  I don't see anything related to TLS, SSL,
etc., or anything else that looks like it might be connected to this.
I can post the list of packages that have been upgraded if that seems
like it would be useful.
 
I'm not sure how to get more information on this problem and don't have any
idea where else to look.  Suggestions?

Thanks.