That's a good question, but it's so obvious that that's almost the point. So, to look at it another way, this isn't something anyone really worries about today with gmail et al. I agree.
But if you host your mail on your cloud server, or even in your home, wouldn't you feel especially worried if you saw that you had received such illegal material? What do you even do? Do you wipe the server? How many logs do you keep just in case? Do you keep them for 5 years? 10?
If you feel even slightly differently about those two options, that's the concern that is still unresolved for me. If you honestly don't, after having thought about it, then I would consider that a valid point of view here.
One reason I would feel safer with larger providers is they would be better-equipped to block the content, better-equipped to deal with the authorities, and liable for the infrastructure itself. If the cops come to confiscate or investigate some of the hardware, they're going to be dealing with Google. They may also deal with you, but if you're savvy enough not to download attachments you don't expect to receive, you may have SOME buffer there. If it's clear you are hosting your own mail, you have absolutely no buffer. It's very likely to get all your home/cloud computing equipment seized. Maybe THAT's the part that's not realistic?