I'm not sure why a trigger is limited to no more than 30 days after check-out. I'd really like to send my guests some follow-up messages later than that, inviting them back and giving them a special offer. Can you not let us create further-out triggers?
That's because messages within 30 days of a past sales interaction are considered to be related to that sales interaction, and thus are not spam or marketing. Allowing emails longer than that would add additional legal requirements and complexities. It's cleaner for OwnerRez to be able to say "no, we just don't allow automatic emails like that at all".
Of course, you are perfectly free to export your guest list to Excel and use it in another system that is designed for general email marketing - there are many of them around with good reputations.
At some point in the future, we'll probably add more capabilities here, but because of the growing legal restrictions, this would be more complicated than it might at first appear.