Thunderbird/kmail - how not to preserve messages?
 

I use Thunderbird on my Linux Kubuntu 11.10 instead of KMail though both are installed and configured. However, when I receive mail with TB they are not deleted and KMail also can receive them, not vice-versa. What option should be set in order that TB deleted received messages.

It sounds like you are using a POP3 mailbox and TB is configured to 'leave mail on server' so that KMail is also then able to collect the same messages. Look for the option to delete mail after collection within TB and this should ensure that your POP3 mailbix is emptied after collecting mail in TB.

1. A little bit strange, but TB treats my mail server as IMAP and KMail as POP3 (the same address indeed). TB has set this automatically while configuring my account first time .
2. I can't find an option you mentioned in TB. There is, however, such in KMail.

Anyway, no step forward.