[Dovecot] lmtp-postlogin ?

We have last-login tracking for imap and pop, and I intend to use this for deciding which users to backup daily. But, it would also be nice to backup users who has only received messages, but not logged in lately.. So is it possible to implement "last-login" tracking for lmtp ?

I naively tried copying the settings from imap, but it didn't work:

service lmtp-postlogin {
  executable = script-login /usr/local/sbin/lmtp-postlogin.sh
  unix_listener lmtp-postlogin {
  }
}

service lmtp {
  executable = lmtp lmtp-postlogin

<snip>

-jf