Intermittent failure to send and receive btinternet.com emails.

I use Windows 11 Pro on a desktop computer. I use Firefox (which is up to date) and Thunderbird as my email client (which is up to date). I use Zone Alarm Extreme Security as my Firewall and Anti Virus Recently it has become a nightmare sending and receiving emails to my markhowes@btinternet.com email address. Sometimes the emails (which I know are on the bt server) don't arrive but I don't get an error message. Then later in the day I might receive loads of emails at once. Sending emails is even more of a problem. They simply don't go. Sometimes I get a time out message, sometimes I get an incorrect password message. Within Thunderbird there is an option to check connection to both the outgoing and incoming bt servers. Usually (but not always) connection shows as "successful". This has been going on for a couple of months - maybe it's a coincidence but it seems to start up after a Windows automated update and then things settle down again (I think) until the next Windows monthly update. I'm not all that computer literate so please don't suggest messing around with the Registry or other very technical solutions but if anyone can help me solve this maddening problem I would be really grateful. I can send and receive emails on my Android phone and directly on the bt email site via my Firefox browser. Mark