Is it possible to make Postmaster update an entry's custom field after it sends an email? I've got a custom field called 'Email Sent' and it'd be nice to stick a 'yes' in there for peace of mind once the email had gone out.
That's a great idea. Currently there are no hooks or settings in place to do that.
I could also see a setting in place to store the number of times an email has been sent (for re-occurring emails). That way, you could say do not send after X times.
I will add this to the list of features to implement. If you need this immediately, I am available for an hourly rate of course. Email support[at]objectivehtml.com if you need such services.