Read an email address in two parts
In alex@example.com, alex is the local part and example.com is the domain. The complete address identifies a mail destination; the domain is shared by other addresses such as sales@example.com.
The provider’s configuration determines whether each destination is a mailbox, alias or another supported route. Two addresses with the same domain do not necessarily represent two independent accounts.
Understand the website address
A website might use example.com or www.example.com. The www portion is a subdomain. Visiting that address and sending mail to alex@example.com involve different services and different DNS records.
Registering example.com gives you a registration for that domain under the applicable terms. It does not, by itself, build a website, create mailboxes or configure a secure login for employees.
Separate the service roles
A registrar manages the domain registration. A DNS service publishes the records used to locate services. A website host serves the site, and a mail provider handles email. One company may supply several of these roles, or you may use separate companies.
Write down which company performs each role for your business. This avoids contacting the website designer about an email outage when the designer has no access to the mail provider.
Know what changes affect
Changing the website’s address records does not require replacing working mail records. Changing nameservers, however, changes where the domain’s DNS is managed; ensure all required records exist at the destination before a switch.
For a fictional business, the handover could read: domain registrar owned by the business; DNS managed by its administrator; website at one host; mail at another. Keep account recovery independent where practical so one outage does not remove every recovery channel.
Ask for an explicit purchase list
Before paying, list domain registration, renewals, website hosting, mailboxes, sending capability and support. Mark each as included or separate. An offer called a domain package is not sufficiently precise to establish what your team will receive.