Business
Moving one mailbox is an evening of technical work. Moving forty is a week of coordination in which the technical part takes the least time of all.
A single migration forgives improvisation: wrong click, restart, forget. Across forty mailboxes every small thing is multiplied by forty, and a technical slip turns into an organisational one.
Three problems show up every time: nobody has the exact list of mailboxes, some passwords are unknown, and staff learn about the move at the moment their mail stops working.
A sequence that holds up across dozens of mailboxes. The first two steps take longer than the migration itself and save the most.
A note the day before the switch removes most of the support calls. It needs four things: when it happens, what to do in their mail client, that the old mail is already there, and where to write if something looks wrong.
Two normal side effects are worth mentioning up front: messages may re-sort by the date the new server received them, and mobile clients will ask for the password again.
A company migration does not have to be a cloud one. If policy forbids sending mailbox passwords outside, a local client does exactly the same work on your own machine: credentials and mail never leave the perimeter.
That also answers the volume question: a local migration has no gigabyte cap, and forty mailboxes of ten gigabytes each is an ordinary case.
FAQ
Technically yes, practically no: providers cap simultaneous IMAP sessions, and batches of three to five usually finish sooner than everything at once.
Migrate them as an archive and leave them closed on the new server: the mail stays available to the company and nobody reads the mailbox.
You need IMAP access to each mailbox. In practice that means app passwords, issued by the administrator or by each user.
Estimate from the largest mailbox and add headroom for batching. Provider speed figures are in a separate article.
A separate password for IMAP instead of your account one: who requires it, where each provider keeps it, and what to do when the section will not appear.
Speed is capped by the two mail servers, not by the software. How to estimate the time in advance and what to expect from a mailbox of tens of gigabytes.
Labels versus folders and the All Mail folder: the number one cause of doubled volume and duplicates, and what to exclude before you start.
Connect two servers, look at the size and decide. Or take the client and keep everything on your side.
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