I have a webmail with 90.000 imap accounts I'm trying to delete old email accounts (for example : accounts that has not been used for 1 year)
this link may help to best answer: http://www.dovecot.org/list/dovecot/2011-January/056320.html
os : centos 6 64bit
The link you posted describes an approach that you can use in the future to gather information about the last login date. That is necessary because by default, no information regarding the last use is logged by the system in a persistent manner.
That means that if you want to delete accounts not in use for a year with this method, you can do it only in a year from now.
However, if you don't understand how to follow the very clear and easy instructions in the post you linked, I have no real idea to help you further and frankly, you have no business of running a mail server for 90k users.
An attempt to make this a bit clearer:
What they do is to alter the Dovecot configuration to run a script upon login of a user. This script touches a file in a specific folder named after the user. To touch a file means that it's mtime is updated to today's date. They use this to populate a database where they log who has logged in on every day, enabling them to gather statistics like "who was active at least on 10 days during the last 30 days" etc., but this isn't strictly necessary, you can simply write a script that just iterates over the list of users and delete everyone whose file wasn't updated in at least 365 days.
