I've seen massive of these lines in my /var/log/maillog file, like this:
Sep 21 19:13:55 server26 dovecot[2301724]: auth(
[email protected],185.xxx.xxx.xxx,sasl:plain)<VzJy01I/xoC5nOmi>: da_alternate: missing passwd file: /etc/virtual/domain.nl/passwd_alt
these passwd_alt files do not exist anywhere...