cPanel has the option to force a change of cPanel/WHM passwords on a periodic basis, but there is no implementation currently for email.
I would like the option to force email passwords similar to how it's implemented for cPanel/WHM. I do understand that the biggest issue with this is that many don't use webmail and would need to somehow be notified in their mail client.
I have a couple of ideas:
1. Have the system send them an email at the address letting them know that their password will be expiring soon, and that they need to log into webmail or their cPanel and create a new one. The downside to this is that it could be interpreted as phishing or completely ignored.
2. When the password change needs to happen, sending a verbose error to the mail client. I.e. when they try to check or send mail - the mail server sends an error message to the client letting them know their password is expired and needs to be updated. It may want to include 'Please contact support if you need assistance.
As far as 1. - assuming they check their mail periodically it would work if people didn't think it was spam/junk/phishing.
As far as 2. - This would rely upon the mail client displaying the error to the user - I'm not sure how consistent mail clients are at doing this.
Admittedly I don't write MTAs, POP3 or IMAP daemons, or mail clients - so 2. may not even be an option, or it could possibly be a combination of 1 and 2.
Perhaps others will have more ideas.