Looking for software to monitor scheduled emails and notify when they don't arrive or contain errors

I don't know of anything like this, but it seems like it would be relatively easy to handroll a nagios monitor to check for email in a mailbox and make sure that the last one that matched the specified subject line had happened no longer than X minutes ago. So if you expect to get 1 email/hr, make sure that there's always an email in the mailbox with the matching subject that's less than an hour old. You can check as often as you like, of course since the check is time-independent.