In my experience of setting up mail servers in the corporate world, often it is required that users be added to external databases which could be part of some strange enterprise applications. It could be as simple as adding users to your ldap server when creating a mailbox on IndiMail. Sometimes it could be as bad as adding users to ADS (ugh).
IndiMail (release 1.6.9 onwards) provides you a hook, to execute any program after successful completion of the programs, vadddomain, vaddaliasdomain, vdeldomain, vadduser and vdeluser, vrenamedomain, vrenameuser, vmovuser, vpasswd.
A hook can be defined by creating a script or an executable in /usr/libexec/indimail with the name of the program being executed. e.g. if you create a script named vadduser in the directory /usr/libexec/indimail, the script will get executed whenever the program vadduser is used to add a user to indimail. The execution happens only if the program completes successfully. Depending on what you need to do, you can customize the scripts in a jiffy.
The hook script name can be overridden by setting the POST_HANDLE environment variable.
See the man pages of vadddomain, vaddaliasdomain, vdeldomain, vadduser, vmoduser, vmoveuser, vdeluser, vrenamedomain, vrenameuser, vpasswd.
for more details.
Let me know if you create an interesting script.
Example of using a handle is when adding a user, you want vuserinfo to be run.
If you have the above, then this is what will happen when you add a user
I personally use post execution handle for adding some mandatory users every time I add a new domain. So this is what my vadddomain handle looks like