Email is intrusive, and why
Here is something that I've alluded to before: email is quite intrusive (well, at least in my branch of the world). For both technical and social reasons, email messages are far more like (work) phone calls than they are like paper mail.
On the technical side, you get a lot more email than you do paper mail, and it's harder to scan and sort your email than your mail. For example, most paper mail is loaded with cues as to what sort of mail it is, ones that are relatively easy and reliable (at least until scammers start exploiting them by sending out offers that look like invoices). Yes, paper mail would be more difficult to deal with than email if we got as much paper mail as we did email, but we don't; as an aggregate, paper mail is thus less of a burden.
Socially, there's a strong expectation that people will read their email frequently and respond to it fairly rapidly (although this varies a lot from group to group, and I happen to be in a group that expects this). This expectation creates a social pressure to actually be this responsive, and the more responsive you are the more intrusive email is.
(Email is not alone in this. Any medium where you're expected to be responsive is necessarily going to be intrusive.)
This holds true even if you carefully label your email messages as low priority. Unless the recipient's software can automatically sort them into some 'go through once a day' category, they still get interrupted so they can determine if the message is important or not.