[JohnR]

Thunderbird has been giving me a lot of error messages about being unable to connect to the mail server. The messages still come through, eventually, but it's been a bit worrying...

Then I realized that I few days ago I had installed Avast for Mac because I had gotten another Chrome adware virus . Could there be a  connection.... ? So I uninstalled Avast, and Thunderbird is immediately just fine again.

Avast didn't detect the adware virus, BTW. Removing all extensions, clearing everything and setting it back to defaults seems to have solved it. I've shifted back to Firefox for everyday browsing though, Chrome seems prone to this problem.

Update: I've just discovered that this is a known issue and there is a way tol solve it (it didn't show up in my earlier searched on the error message, at the time I hadn't connected Avast to it):

https://www.avast.com/faq.php?article=AVKB91

[GentleBender]

I've not been a fan of Chrome, too many issues dealing with it at work. I've not had any problems with Safari or Firefox that I have needed Chrome. Have you tried Kaspersky on the Mac yet? I still don't run virus protection on my Macs and have been fortunate so far, but I practice caution and mainly stick to my known sites. That doesn't mean they can't get injected by malware... Rush and roulette!

[JohnR]

I've not tried Kapersky. I've never felt I needed antivirus on the Mac either and as you I am fairly careful. I found Avast by googling for adware removal tools.

At this point, I think the best solution is to not use Chrome  I could be wrong, maybe I'll get something again on FF or Safari. I don't remember why I switched to Chrome some years ago.

[GentleBender]

There are quite a few add-ons for Chrome, but to me they just add more surface area for malware and adware to attack. I run little to no add-ons. Add-ons can make things easier, but one bout with a nasty virus takes all that time and effort saved and can really ruin a day or week. Some people lose a lifetime of pictures and documents or worst. I'm a little paranoid...if I want to do some surfing in choppy waters, I use my iPad.

[JohnR]

I suppose I should update my original post as I've just discovered that this is a known issue and there is a way to solve it (it didn't show up in my earlier searches on the error message, at the time I hadn't connected Avast to it):

https://www.avast.com/faq.php?article=AVKB91