Continue to use whatever you are using, but add SSL/TLS to it via a
local proxy, like sTunnel.
You configure your e-mail client to connect to the sTunnel local proxy.
You configure sTunnel with the server settings you were using before in
your e-mail client.
email client -> sTunnel -> server
https://www.stunnel.org/
https://www.stunnel.org/platforms.html
According to the last doc, sTunnel runs on Windows XP.
sTunnel runs continuosly in the background; i.e., it is a resident
process that loads on your Windows login. I used it long ago when
whatever I was using back then didn't support the TLS needed to wherever
I was connecting. Haven't needed it in many years, though, after moving
to an e-mail client that does have TLS support. Currently I'm using:
eM Client (free up to 2 accounts, paid for more account connects)
I'm using it on Windows 10, and Windows 7 before that. The current
version runs on Windows 7, and above, but their FAQ notes:
Windows 7 or higher (for XP compatible versions contact our support -
although it is no longer a supported system, so we do not recommend
its use).
You are used to using Outlook Express. There is OE Classic (not from
Microsoft) that should be familiar to you, and it supports Windows XP.
There is a free version (OE Classic) and paid version (OE Classic Pro).
They don't operate peer forums for community help, and I've not used
that program to know how responsive and helpful are their tech support.
I suspect you only get tech support if you pay. They offer a money-back
promise for 60 days on the Pro version. You can start with the free
version. If you hit too many features you want available only in the
Pro version, decide then if you want to pay, or stay crippled. The free
version might be all you need.
NOTE: I see you discussed OE Classic in another subthread, and noted
it watermarks your outbound e-mails. I doubt the paid version of OE
Classic Pro does that.
Hexamail Flow advertises itself as the "e-mail cliet for XP". Free for
personal use, $19 for business use.
https://www.hexamail.com/product/hexamailflow/
Their features page notes "Outlook Express Import". Since you are using
POP, your e-mails are stored locally instead of up on the server (as
with IMAP) unless you configured OE to *not* delete after retrieve
(don't remember if that's an option in OE). Normal POP issues a RETR
followed by a DELE.
As you noted in the other subthread, you could try using an ancient
version of Thunderbird hoping it supports TLS connects. Firefox 52 ESR
was the last version that supported Windows XP, and Tbird is a fork of
Firefox.
EmailTray says it supports Windows XP. Never tested that program. You
would have to find out from them if they support TLS connections to mail
servers, and if they support OAUTH2 (for Gmail, Hotmail, Yahoo). You
might online reviews by searching on "emailtray review".
https://www.emailtray.com/