If they're going to run their own email server, yes, otherwise they have the easy choice of pointing the mx record for their domain at office365, Google, fastmail or another third party service that will handle all the complicated smtp administration for them.

Either doing their DIY properly or outsourcing it for $8/mo to someone who can do it properly says they actually care about mail delivery working, and their domain not ending up in peoples' spam traps (ex: try sending mail to someone from a domain with no spf record).