Igor, I do not beleive that the Avast Icon is spinning on mail deletion but I will check later when I get home. I only have at max up to 15 mail messages at any given point in time since I try to keep it clean. I can even delete one piece of mail that has no attachments on it and only a few lines of text but the result is the same. I have not tried the exclusion list on it, but I know for a fact that when using Outlook, it does not matter how much mail is in there or what the standard shield is set at, the deletion takes place immediately and a long delay when using Thunderbird except when I set standard shield to scan default files only.
I believe Thunderbird stores mail in the mydocuments/username/application data/Thunderbird/profile folder. I can also try to exclude it and see what happens. I have tried a few other e-mail programs also and have had no problems with any of them except TB.