Automated systems are supposed to send to Sender, except when they're not... well, it depends on what you mean by "automated". In the "classical" sense, the only automated system was the software that implemented SMTP itself, such as sendmail. It generated bounces, it generated vacation replies, it was both the point of mail ingress and egress for mail's state in the system.
Now, there are automated systems that exist outside of SMTP. For example, mail user agents generate vacation replies. Things like gmail and Exchange/Outlook blur the line between mail server and mail user agent. SMTP generated bounces are rarely useful for humans, but things like vacation messages are intended for humans. If there is a reply-to, that's assumed to be more of a human (a human who will at least read the response, bounce or otherwise) than the chances that the Sender (on a message with a Reply-to) is going to be a human.
So, if you're talking about bounces, I'd expect them to go to Sender. For vacation messages, I expect them to go to Reply-to. And of course, YMMV, even MMMV, based on the email topology of any specific organization. What counts as a bounce? What counts as a auto-reply about the availability-state of the recipient? Or the availability-state of the person/role collecting the recipient's email? Is that significantly different from a bounce? Can an automated system determine that? Should it?
(sidenote: Personally, I hate vacation messages. People think it provides an explanation for not immediately replying (anyone who thinks email is reliably immediate is an idiot), but all it does is show you care more how you don't respond when you're not around than when you are.)
Now, there are automated systems that exist outside of SMTP. For example, mail user agents generate vacation replies. Things like gmail and Exchange/Outlook blur the line between mail server and mail user agent. SMTP generated bounces are rarely useful for humans, but things like vacation messages are intended for humans. If there is a reply-to, that's assumed to be more of a human (a human who will at least read the response, bounce or otherwise) than the chances that the Sender (on a message with a Reply-to) is going to be a human.
So, if you're talking about bounces, I'd expect them to go to Sender. For vacation messages, I expect them to go to Reply-to. And of course, YMMV, even MMMV, based on the email topology of any specific organization. What counts as a bounce? What counts as a auto-reply about the availability-state of the recipient? Or the availability-state of the person/role collecting the recipient's email? Is that significantly different from a bounce? Can an automated system determine that? Should it?
(sidenote: Personally, I hate vacation messages. People think it provides an explanation for not immediately replying (anyone who thinks email is reliably immediate is an idiot), but all it does is show you care more how you don't respond when you're not around than when you are.)