After switching my nameservers over to Vercel, email stopped working.
This post is about how I fixed that.
How to do it
First, open Vercel's dashboard and go to Settings -> Domains.
Once there, set up MX and TXT records under DNS Records.
Name: @
Type: MX
Value: mx01.lolipop.jp
TTL: 60
Priority: 10
Name: @
Type: TXT
Value: v=spf1 include:spf.muumuu-domain.com ~all
TTL: 60
Setting it up like this should do the trick!