EnterraHost
Most providers move the same way, and this is that route. If your mail is read over IMAP, or has been downloaded by POP3, one of the two sections below applies. If you are coming from Microsoft 365, Hosted Exchange or another Exchange-based system, that is a different job and moving your mail from Microsoft 365 is the one to read.
The shape of an IMAP move
Two things happen, and the order matters. First the mail that already exists is copied across while your current service keeps running normally. Then the DNS records are changed so new mail starts arriving at the new place.
Copying first is what makes the move uneventful. Nothing stops working, nobody is locked out, and the only moment with any edge to it is the DNS change at the end.
What you need per mailbox is the IMAP server address of your current provider and working credentials. If your provider uses two-factor authentication, the ordinary password will not be accepted by a migration tool and you need an app password or token instead. Google and Microsoft both work that way, and most larger providers have followed them.
Do not use the customer's real password
If you are moving somebody else's mail, set a temporary password with the old provider for the migration window and give that to whoever is running the copy. Asking a person for the password to their own mailbox is a habit worth not starting, and it makes the migration depend on a secret that may change underneath it.
A password changed mid-migration fails that mailbox and it has to be retried. Keeping batches small is what makes a retry cheap.
If the old setup was POP3
POP3 is different in one important way. Mail collected over POP3 is often stored only on the computer that downloaded it, and the server may hold nothing at all. That means there is no server-side copy to fetch, and the only version of that history is on a machine.
Back it up before anything else. In Outlook that means exporting the profile to a local file first, and in other clients it means whatever their equivalent is. If the machine fails while you are rearranging accounts, a backup is the difference between an inconvenience and losing years of mail.
The move itself is then the reverse of what you might expect. Set up the new account first, get it working, and import the backup into it so the history merges with the new mail. Do not delete the old profile until you have confirmed the import landed.
The one rule that stops a client losing your history
When the new account is ready, add a new account or profile rather than editing the existing one. Changing the server names inside an old profile frequently fails, and worse, it can leave a client that looks connected while your local folders are no longer where you think they are.
Keep the old profile untouched as a reference, add a completely new one with the new server details, and only remove the old one once the new account is verified, syncing, and set as default. That last step is the one people skip and regret.
Where your older mail goes
Folder structure comes across intact, but very large folders are handled differently than you may expect. A folder holding more than about ten thousand messages has its older contents moved into an archive sub-folder, so a long inbox turns into the current mail plus dated archives.
This is deliberate, and it keeps clients responsive and search fast rather than letting a mailbox grow until a data file struggles with it. Active folders keep the current year and the previous one.
It does surprise people after a migration. If a folder looks emptier than it should, look for the archive sub-folder underneath it before assuming anything was lost, because nothing was.
The DNS change
Where you change the records depends on who manages the domain. If the domain is registered with us, your DNS lives in the toolbox and you edit the records there. If it is still with another registrar or DNS provider, the change happens in that provider's own panel, and you can either make it there or transfer the domain to us first and manage everything in one place afterwards.
Lowering the time-to-live on the old records before you change them is worth the small effort. It shortens the window in which some senders still reach the old service, and it means a mistake is undone in minutes rather than hours.