Running OS-level programs from Postgres?

Maybe this is a well duh kind of question, or maybe there's no
straightforward way to do it, but is there any way that I could have a
pg function initiate a process on the host system?

Specifically I'd like to script an email to send off on an insert
trigger, but the ability to initiate system processes in general seems
like it would come in handy.

My present need is for email notification; if there's a pg function or
module that would handle this (I haven't turned up anything in my
searches, but maybe I'm using the wrong search terms in the wrong
places) I'd be OK for now, but I'd rather have the flexibility of
starting whatever process a given circumstance calls for.

Thanks.