[Dovecot] mysql log entry

I'm sure this is a really newbie question, but I notice in /var/log/mail.log that there are between 3 and 15 of the following log entries every day for the last five days, which is far back as my logs go--what do they mean?

Apr xx hh:mm:ss mail dovecot[109]: auth-worker(default): mysql: Connected to localhost (postfix)

Is it dovecot connecting to mysql for password or user info? If so, wouldn't it be more frequent? I've got 20 users with some connecting from outside many times per day.

Thanks, -Terry