information on how to setup an email domain is easy to find but sometimes its not that easy to visualise a stepwise procedure involved, so I have broken down the whole setup process into the below 12 refinements for some clarity. The below process also assumes you are using Mimecast as the email hygiene/gateway solution.
1. Choose you suggestive domain for example mydomain.co.uk – setup mydomain.co.ukdomain on-prem as authoritative on-prem in Exchange
2. Add domain to AD domains & trusts so UPN can be added for user identity.
3. Add a user account on-prem and sync to O365 with correct UPN
4. Give user an O365 license inclusive of the Exchange license
5. Add domain to O365 and go through verification process
MX record set priority to 5
TXT record add
CNAME record add
Set correct SPF record (if routing email via a 3rd party provider this will be their SPF for example Mimecast or Barracuda if routing through their servers).
6. Make sure user has correct primary UPN in 365
7. May require another Azure AD Sync at this stage.
8. Add domain to 3rd party email hygiene provider and will probably require DNS verification and will need to perform Directory Sync again (not needed if not routing via 3rd part such as Mimecast)
9. Check Mimecast – gateway – policies – routing and ensure the domain will not route directly to on-prem exchange and must go to Exchange Online.
10. Setup DKIM or DMARK for domain if required
11. Remove any old domain or DNS records
12. Test mail flow