ip switching from ipv4 to ipv6

[re]

dear ipv6-ops list members,

one of our users is unable to login to webmail, because the ip address is
changing during session. this problem isnt that unusual, but after a check
in the logfiles i realized its switching from ipv4 to ipv6 and so on, (see below).

is this phenomenon known?
is it us or the deutsche telekom who is causing the problem?

thanks in advance,
re - noc.mur.at

#####

Apr 29 09:54:34  dovecot: imap-login: Login: user=<>, method=PLAIN,
rip=217.233.160.66

Apr 29 09:57:54  dovecot: imap-login: Login: user=<>, method=PLAIN,
rip=2003:88:6c55:4707:b538:efe9:d4fb:bdf7

Apr 29 10:05:17  dovecot: imap-login: Login: user=<>, method=PLAIN,
rip=217.233.160.66

Apr 29 10:05:19  dovecot: imap-login: Login: user=<>, method=PLAIN,
rip=2003:88:6c55:4707:b538:efe9:d4fb:bdf7

Apr 29 11:29:03  dovecot: imap-login: Login: user=<>, method=PLAIN,
rip=217.233.160.66

Apr 29 11:45:09  dovecot: imap-login: Login: user=<>, method=PLAIN,
rip=2003:88:6c55:4707:b538:efe9:d4fb:bdf7

-- 
--mur.at leitnergasse 7-12 8010 graz--