multiple IMAP sessions when connecting from one client

Hello,

I am using Postfix/Dovecot/IMAP.

Everything is working fine, but I have noticed that every time I connect with Thunderbird to my server via IMAP, not one but 4 connections are being logged into /var/log/mail/mail.log:

2014-08-27 09:17:46 dovecot: imap-login: Login: user=<martin>, method=PLAIN, rip=11.22.33.44, lip=55.66.77.88, mpid=12519, TLS, session= 2014-08-27 09:17:53 dovecot: imap-login: Login: user=<martin>, method=PLAIN, rip=11.22.33.44, lip=55.66.77.88, mpid=12521, TLS, session= 2014-08-27 09:17:53 dovecot: imap-login: Login: user=<martin>, method=PLAIN, rip=11.22.33.44, lip=55.66.77.88, mpid=12523, TLS, session= 2014-08-27 09:17:54 dovecot: imap-login: Login: user=<martin>, method=PLAIN, rip=11.22.33.44, lip=55.66.77.88, mpid=12525, TLS, session=

This looks like 4 separate sessions are created when I log in from one client.

Is this normal behavior? Can somebody please explain why this is so?

My Dovecot version is 2.1.7

regards, Martin