IMAP to Exchange Online migration

IMAP describes folders of messages and nothing else. Calendars, contacts, server-side rules, delegated access, aliases and out-of-office settings are outside the protocol, so no IMAP tool can move them regardless of vendor claims. They are exported separately, usually as ICS and CSV from the source platform's own interface, and imported after the mail is in place.

This is worth stating plainly because it is the biggest source of disappointment in low-cost migrations. The mail arrives, everyone relaxes, and then the first Monday meeting is missing from every calendar.

Exchange Online will accept mail far faster than a shared hosting server will give it up. The binding constraint is nearly always the source: connection limits per account, total concurrent IMAP sessions, CPU on a shared box, and throttling that some hosts apply the moment they see sustained sync traffic. Set the plan from observed pilot performance and source-provider limits, not a universal throughput assumption.

The practical consequence is scheduling. Pre-sync starts days before the cutover, runs at a concurrency the host tolerates, and is followed by short delta passes. Trying to compress an IMAP migration into a single weekend is how estates end up with half-copied mailboxes on Monday.

Older IMAP servers use folder separators and naming conventions that Exchange rejects. Slashes and dots in folder names, folders nested deeper than Exchange permits, characters that are legal on the source and illegal on the destination, and Sent or Trash folders under half a dozen different names. Each is trivial individually and collectively they account for most of the errors in a migration log.

We normalise these in mapping before the first pass, so the log stays readable and genuine failures are visible rather than buried under hundreds of naming warnings.

Hosted IMAP accounts often carry a decade of mail because nobody ever enforced a quota. Exchange Online mailboxes have plan-dependent ceilings, so an estate with several 60GB accounts needs archive licensing or a retention decision before any data moves. We surface the distribution of mailbox sizes in discovery so that purchase decision is made once, correctly.

Where an archive is the right answer, mail beyond the cut-off is written to the online archive during the migration rather than moved twice afterwards.

IMAP source limits can apply per user, IP or server and can change during a migration. Request the host’s limits and run a pilot at agreed concurrency; a universal 1.2 GB/hour figure is not a measured estimate for your source.

Inventory calendars, contacts, aliases, delegates and any local-only mail separately from IMAP messages. Compare message counts and exceptions after each pass, then verify inbound and outbound routing before ending source access. Source overlap reduces risk but does not guarantee every sender retries.