Roberto,
The problem you report is differnt than the symptoms of Thunderbird's
connection problem.
I do not know if Cyrus has this setting. However, if you set
Thunderbird's max to 1 you can be sure Cyrus' connections will be >= to
1. Check your permisions on the Send directory (if you use maildir) or
equivalent if you don't.
Thunderbird will work fine with 1.
That was not it :-(
If I set thunderbird to copy to the "Sent" folder, it hangs on the
"copying to sent messages folder" dialog. Also, cyrus uses its own
mailbox setup. I.e., not mbox or maildir.