A terabyte count is a useful number. It is rarely the number that decides whether an email archive migration finishes cleanly.
The work usually stalls in the gaps between platforms: what the source will actually give you, what the destination will actually take, and what happens to the messages that fit neither story.
Start with the estate, not the slogan
Archive estates are uneven. Some mailboxes are tidy. Some are journals, shared mailboxes, legacy exports, or years of retention nobody has opened since the last platform change. A single “about 40TB” hides all of that.
Discovery is how we get a usable picture before delivery starts. Size, data shape, access, retention constraints, and the exceptions that are already visible. Better facts make a better plan, a better quote, and a migration window that survives contact with the source.
We do that work on the eMXport platform, which we deploy and operate for the engagement. Your team does not install anything.
The long tail is the schedule
Happy-path messages move. The schedule is set by the ones that do not.
Throttling. Holds. Items the destination rejects. Files that are not the format anyone expected. Batches that need a human decision. If those turn up in week six, the plan you agreed in week one is already fiction.
That is why we would rather measure the awkward bits early. Exceptions are not a surprise category. They are part of the job.
Done is a record, not a progress bar
A migration is not finished because a counter reached the number you hoped for. Someone still has to explain what was processed, what was excluded, and how the job closed.
We agree that reporting during discovery, then hand over the chain-of-custody record defined for the engagement. The exact package depends on the route, and on what your technical, project and compliance teams need to see.
If you already know the source, the destination and a rough size, that is enough to start.