When the link between two LANs comes up, you may well want to make sure that email that is queued at either end is flushed - sent out to its destination. This is done by adding the appropriate sendmail invocation.
Using the bash 'case' statement on an appropriate parameter that pppd passes into the script accomplishes this. For example, this is the /etc/ppp/ip-up script I use to handle our WAN links and the link to my home Ethernet (also handled on the same ppp server).