The Philosophers' Cocoon

In our newest "how can we help you?" thread, a reader writes:

I'd like to push the topic of norms for correspondence. My question is simple: Under what circumstances (if at all) is it acceptable not to reply to an email. I don't mean only requests to comments, but also various invitations to conferences, to contribute to an edited book, to give a lecture etc.

My view is that it is impolite not to answer such an email, even a weird one. It takes less than one minute to write "Sorry, I can't do that."

Good query. I've often been quite surprised here. For example, I've heard stories of philosophers who committed to contribute to an edited volume deciding later on to never respond to emails–essentially 'ghosting' the editor they had an agreement with. This seems to me profoundly unprofessional. Similarly, I've even heard that some journals routinely do this: ignoring repeated emails from authors to check in on the status of manuscripts after, say, 8 months of review. I find it astonishing that there are journals that do this kind of stuff. I don't understand why authors don't protest them by refusing to submit work.

I was always taught that basic norms of professionalism dictate responding to emails within 24 hours during the work week, so that's what I generally try to do (I do not normally respond on weekends). But there are some kinds of emails that I may not respond to at all, or only after some delay. For example, I constantly receive unsolicited requests to 'guest post' on the Cocoon by non-philosophers looking to see this or that product. I don't respond to these. And, of course, like many philosophers (or so I've heard) I sometimes receive emails from non-philosophers who claim to have solved every philosophical problem ever, asking me if I'd care to read their 400-page book. Generally speaking, I usually respond kindly to these kinds of queries, politely declining. But aside from these kinds of things, I try to get back to everyone who contacts me fairly promptly. 

But this is just what I think and do. What do you all think? What norms should one follow for correspondence? Under which circumstances is it acceptable to not reply to an email?