Merge two mailboxes, or split one in two
Messages are copied, never moved — the source mailbox stays exactly as it is. And nothing is copied twice.
- 1 You give us both mailboxes. The first is the one we read, the second is the one that receives.
- 2 You choose everything, or a slice. Everything is a merge; a slice — by date or by folder — is a split.
- 3 We copy what's missing, and only that. The source mailbox is never modified, and a message already present on the other side is not copied again.
How it works
- You enter both mailboxes — the one we read, and the one that receives. Address, password (or app password) and IMAP settings for each.
- You choose what leaves — everything, everything before a date, everything from a given day onwards, or a list of folders.
- MailSync inventories both mailboxes — headers only, never contents. Each message gets an identity: its Message-ID, or a fingerprint built from sender, recipient, subject, date and size when it has none.
- You see the estimated volume — and start the operation yourself. Nothing leaves before that click.
- Missing messages are copied — into a folder of the same name, created if needed, with their original date and their read, flagged and answered states.
The starting mailbox is never modified. It is opened read-only: even in case of a bug, the server would refuse any write. A split therefore leaves the original mailbox complete — you check the copy, then you decide.
No duplicates created. A message already present in the receiving mailbox is not copied again, even if it sits in a different folder there — otherwise merging would create exactly the mess the deduplicator then has to clean up.
Use cases
- Bring two addresses together — an old mailbox you're closing and a new one you're keeping: everything the second one lacks lands there, without overwriting what's already in it.
- Move old archives out — copy everything before a date into an archive mailbox, then lighten the working one once you've checked the copy.
- Take the recent mail to a new address — the mirror of the previous case: only messages after a date follow, the distant history stays where it is.
- Isolate a folder — copy "Clients" and its subfolders into a dedicated mailbox, without taking the rest.
- Consolidate after a host change — when an earlier migration stopped halfway, running a merge only copies what is genuinely missing.
What the tool does not do / limits
- The tool never deletes anything, and a split is not a move: the starting mailbox keeps all of its content. To lighten it afterwards, use the mailbox cleaner, which asks for its own confirmation.
- Trash, spam and drafts are excluded, along with the "[Gmail]" space ("All Mail", "Starred", "Important"), which holds views rather than folders.
- Splitting by folder works from names you type in: an exact name, or that of a parent folder whose subfolders follow. The report lists the folders actually read, so an approximate entry is easy to correct.
- The tool only works over IMAP: mailboxes reachable only over POP3, or through a closed proprietary connector, cannot be processed.
- The volume estimated before launch is that of the whole starting mailbox, not of the slice to be copied: the real figure is often lower, never higher.
- Free up to 5 GB copied. Beyond that, the operation stops cleanly on what has already been copied and can be finished for €5.99 incl. VAT, or with no limit on Premium. This threshold is the same for everyone.
Frequently asked questions
Are my messages moved or copied? +
Copied, always. The starting mailbox is not modified by a single byte — it is even opened read-only on the server, so a write would be refused. A split therefore produces two mailboxes that each hold their share, the first one keeping everything. If you then want to lighten the original mailbox, do it yourself, or with the mailbox cleaner, once you have checked that the copy arrived.
Will merging create duplicates? +
No. Before copying anything, MailSync inventories both mailboxes and identifies each message by its Message-ID (or, failing that, by a header fingerprint). A message already present in the receiving mailbox is not copied again, even if it sits in a different folder there.
How does splitting work? +
By date or by folder. By date: you pick a cut-off day and copy either everything before it (archive the past elsewhere), or everything dated that day or later (take the recent mail to a new address). By folder: you list the folders to copy, one per line; their subfolders follow.
Is my folder tree preserved? +
Yes. Each message lands in a folder with the same name as the one it came from, created if needed. The two servers don't necessarily use the same hierarchy separator ("/" for most, "." for Dovecot): MailSync converts between them.
Which folders are left out? +
Trash, spam, drafts, and on Gmail the whole "[Gmail]" space ("All Mail", "Starred", "Important"), which is made of views rather than real folders. Copying a trash folder across would bring back messages you had thrown away. The report lists exactly which folders were read and which were left out.
Are read and flagged states preserved? +
Yes, on copied messages: the copy arrives in the state the original was in (read, flagged, answered), with its original date. Messages already present in the receiving mailbox are not touched, so their state isn't changed either.
What does it cost? +
Free up to 5 GB copied. Beyond that, the first gigabytes are copied for free, the operation stops cleanly and you can finish it for €5.99 incl. VAT, or with no limit on a Premium subscription. The threshold is the same for everyone.