It is possible but you will have to have a mail server assigned to the domain.

The people using the email may have to change their SMTP values, but they may not.

If you park it with no active hosting though it won't work. There has to be a server setup to handle email requests.

If I were you I would tell them to set up new email addresses and then set up an aliases/auto-forwards and then redirect the email to their new email accounts.... that way people can still email them on their old address, and their email won't clog up your server space. Especially if they are IMAP accounts.

Technically spoken you don't need a hosting account to process their emails. If you don't want to receive any emails on the domain for yourself you could simply point the MX record of that domain to their email server. If you want to receive email on that domain you need to have a simple SMTP server setup somewhere which splits email for the different accounts.

The main problem is not technical but contractual. What if you don't like the domain and you want to sell it in a year or simple abandon it. Will they sue you? What if they send and receive spam or other illegal messages from those email addresses. Will you get caught instead of them? I would think twice before entering such a deal.

A thought: You are paying them for the sale, you are paying the yearly costs for the domain name, you are paying for the email setup to get the email delivered to them. Why should they have the right to receive free emails on that domain for an indefinite amount of time?

This is generally a bad idea if you plan on building a business around this domain. You could offer to do it for a limited time, but at some point the tie needs to be broken. It wouldn't be difficult for him to get a new email address and update his contacts, and meanwhile you set up an auto-responder letting senders know their mail has been forwarded to his new address and to update their records.

And for your own protection, make sure you get a copy of every message that goes through that email account. The last thing you need is the former owner representing themselves as somehow being affiliated with your new company.

digitalv,
Thanks much. Good advice.