Really? Who the fuck cares? If I reach out on twitter nobody bats an eye - but use zachaysan@gmail.com and everyone loses their minds. I used to use my @company email for my startup that was acquired. Now I can't find any email exchanges I know I remember having. I'll use zach@venn.lc for stuffy clients, but I thought YC partners were kinda above this kind of outdated thinking.
Like Michael, I find it convenient, when receiving a lot of email from people I don't know well, to have the domain name of their company right in their email address. For example, if I later remember something I want to tell them or someone to connect them to, having their email address autocomplete is nice.
During the early stages of the company email does not get my attention, despite the fact that I know how to set it all up and configure it.
To me this is wise use of time. It would be a waste of time to get properly configured email addresses when the real priority is to get code out there.
It makes it easier for them to immediately look up your company just from scanning their inbox. If they later come across your company separately, it makes it easier to find your message and to follow up on the email thread (they probably don't remember your name).