Making SMTP settings user specific?

I am running ownCloud on my server. It authenticates against an LDAP.
This setup works well. But I do have issues with outgoing notifications (e.g.calendar invitations)

The problem is that my mail server doesn’t allow users to use non-associated email addresses. So, having only one SMTP setting is kind of blocking a multi-user environment.

Is there a way to:
a) set SMTP to a user-specific value/dependant on the user? (SMTP is authenticating against LDAP as well, so there is a chance to re-use it?)
b) change the sender email address per user (they have a default email address for internal use)

Not sure if this is configurable or if there is a plugin for this?