Tenant to Tentant Migrations - Which Third-Party Providers?

OllyLennox Tenant to Tenant migrations are a bit tricky because they require moving the domain.  Microsoft only allows a Domain to live in one tenant at a time.  So to move the domain you need to release (remove) it the current tenant in order to move it across to the new tenant.  Removing the domain requires disassociating the domain from all User Names (UPN), email addresses, (including) aliases, Distribution Groups, Security Groups, O365 Groups. Public Folders, Azure Applications & Sharepoint (you get the idea).  Typically most orgs will change all domain associations to the tenant .onmicrosoft.com domain. i.e mailto:bob@bobsautoparts.com to mailto:bob@bparts.onmicrosoft.com. Now if you have some PowerShell gurus to do all of this, it is laborious and can result in some extended email downtime.

To move the data efficiently with least amount of potential email interruption, most orgs will create new mailboxes at the destination on that tenants .onmicrosoft domain.  Do an email sync of existing email prior to all the moving of domains.

Second piece is obviously adding the domain to the new tenant and getting everything switched over to the moved domain.  And again you would need your PS wiz to flip all the email addresses to their original designation.

Don't forget if you use Outlook for email, your users will need to create brand new profiles for all desktops, laptops, phones, tablets, etc.  Even though the address is going to be the same, the mailbox has moved to a new exchange GUID and that necessitates the re-connection.

Side Note - SharePoint movement is a complicated beast all unto its own and would require a completely separate methodology, that usually occurs independently of email movement.

Whew - lot of moving pieces there, but it is good have a baseline for researching vendors.

Several choices for companies, but I know several orgs that have used SkyKick with good results. They have pretty solid rep with Microsoft, and a dedicated team to manage this type of email migration.

Good luck with your migration!