multiple IMAP sessions when connecting from one client

[Martin Vegter]

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=<uyb/N5cB+gBUDFRB>
  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=<jioqN5cB/ABUDFRB>
  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=<pqwwN5cB/QBUDFRB>
  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=<cffremmc/gBUDFRB>

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