Ultimately you're looking at it from the wrong direction.
Each directory has one password for each user. ANY service/server that feeds off that directory will use the same password for any given user. It doesn't matter whether you're talking of a GUI login, ssh, FTP, AFP, SMTP, or whatever, they'll all use the same password associated with the username.
If you want a specific server/service to use a different password then you have two options.
One is to run a separate directory server for mail. This directory server has all the same user records but with different passwords. You then configure the mail server to use this directory server rather than the one that all other systems use.
The other option is to create additional user records within the existing server, but with some difference - for example, username 'joe' is your primary account with 'joe-email' as your email username, with a different password. Then you need to configure your mail client to athenticate using 'joe-email' rather than 'joe'. You'll probably also need to setup mail forwarding so that email sent to 'firstname.lastname@example.org' ends up in the joe-email mailbox.
In both cases you'll increasing your workload by maintaining either two directories or two records for each user. You'll have to decide how much effort that is, and whether it's worth it.
So you are trying to sneak into the private life and communication of your kids?
I don't know how old they are neighter this makes a difference to the rights of ones private life or if it's legal where you are living.
Don't you mind they are loosing their trust in you if they find out that you are lurking over their private communication?
If you are afraid they could become victims of Internet crimes or pedophils you better try to explain them the risks of the Internet so they will ask you if something is wrong rather to control them without their knowledge.