Gmail

Why a Gmail migration doubles your message count

12 Sept 2026·3 min read

Someone moves a ten-gigabyte mailbox and finds twenty on the other side. Nothing broke: Gmail presented the same messages twice, and the migration faithfully copied everything it was shown.

Gmail has no folders

Inside Gmail a message exists exactly once and labels are tags on it. A message with three labels is one message shown in three places.

IMAP has no such concept. It has folders, and a message lives in a folder. To expose labels to an external client, Gmail presents every label as a folder and adds a system folder called All Mail that contains everything.

The arithmetic follows. Any migration tool sees the message in the label folder and the same message in All Mail, and copies both, because as far as the protocol is concerned those are two messages in two folders.

What the Gmail folder list looks like over IMAP

* LIST (\HasNoChildren \All) "/" "[Gmail]/All Mail"
* LIST (\HasNoChildren \Trash) "/" "[Gmail]/Trash"
* LIST (\HasNoChildren \Junk) "/" "[Gmail]/Spam"
* LIST (\HasNoChildren) "/" "Clients"

What to do about it

The answer depends on what you want on the other side: the label structure, or simply all the mail as one archive.

You want the structure — exclude All Mail, Spam and Trash. Inbox, Sent and your labels arrive as ordinary folders.
You want a flat archive — migrate only All Mail. One large mailbox, but guaranteed free of duplicates and of gaps.
What you do not want is both at once: that is the doubled volume.
A message with no label at all lives only in All Mail. Exclude that folder entirely and archived mail does not come across — the flip side of the first option.

Measuring the real size in advance

The figure in Google account settings covers Drive and Photos too, which makes it close to useless for planning a mail migration.

Look at folder sizes over IMAP instead: the sum of every folder except All Mail is the structured option, and the size of All Mail alone is the archive option. The gap between those two numbers is what the decision costs.

Run a folder-size pass without copying anything.
Compare the sum of your own folders with the size of All Mail.
Pick an option and exclude the extra folders before the start, not after.
Compare per-folder message counts once the migration finishes.

Other Gmail surprises

Messages in Sent often carry a user label as well, so they land in both places. Same mechanism.

Important and Starred are IMAP folders too, and they are copies as well. Nearly every migration excludes them along with Spam and Trash.

FAQ

Common questions

Can Gmail labels be migrated as labels?

Only when the receiving side is also Gmail. In plain IMAP a label becomes a folder, so a message with three labels lands in three folders.

Will a second run create duplicates?

No. A repeat pass compares the destination and copies only what is missing.

If I exclude All Mail, do I lose the archive?

You lose messages that carry no label and are not in the Inbox. If you rely on the archive, migrate All Mail and exclude the labels instead.

Why does the size still differ on the new side?

Providers account for metadata and attachments differently. Compare message counts per folder, not gigabytes.

Where to next

Move from Gmail to Outlook Move from Gmail to Microsoft 365 QUOTA EXCEEDED explained

Move your mail with MoveMailbox

Access4 min read

App passwords: why you need one and where to get it

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.

Planning4 min read

How long does moving a mailbox take

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.

Hosting3 min read

Moving mail when you change hosting, step by step

One rule: mail first, DNS second. The full sequence, including TTL, the catch-up pass and the window when messages land in both mailboxes.

Find out how big your mailbox is. It costs nothing.

Connect two servers, look at the size and decide. Or take the client and keep everything on your side.

Start the transfer Download for Windows

Code on GitHub · built on imapsync · no sign-up