Folder separators: Dovecot / versus Courier .

In IMAP, nesting is carried by a string rather than a structure: Clients/2025 is the folder 2025 inside the folder Clients. Which character separates them is the server decision, and the protocol allows that.

Dovecot and most modern servers use a slash. Courier has always used a dot. Older Cyrus builds use a dot as well, and Exchange gateways sometimes present a backslash.

A client is supposed to ask the server for the separator with LIST and use that one. Skip it — as simple scripts and some in-house tools do — and the folder name travels to the other server unchanged, where it now means something else.

Three typical pictures that give the diagnosis away immediately.

A proper tool asks both sides for their separator and rewrites names as it copies: slash to dot, dot to slash, adding or stripping the INBOX prefix where it belongs.

That is what folder auto-mapping means. In the overwhelming majority of migrations it does exactly the right thing with no intervention.

A manual rule is worth it in exactly one case: when a folder name contains the character that serves as the separator on the receiving side. Then you replace it explicitly — the dot in Acme Ltd. Contracts with a hyphen, say — because otherwise any automatic rule will create nesting that was never there.

A minute spent on the folder list saves an evening of cleanup after.