Protocols
It sounds like a choice between two options, but there is no choice. POP3 exists to pull mail onto one computer and free the server; IMAP exists to work with the whole mailbox. A migration moves the whole mailbox.
POP3 knows exactly one folder: the inbox. Sent, Drafts and your own folders do not exist as far as the protocol is concerned — it simply never shows them.
It keeps no per-message state on the server. Read, answered, flagged: all of that lives in the client, not in the mailbox, and disappears with the client.
And the big one: classic POP3 behaviour is to fetch a message and delete it from the server. Most clients now ask to keep a copy, but that is a setting, not a property of the protocol.
IMAP shows the mailbox as it is: the full folder tree, flags, dates, attachments, headers. A migration over it copies structure rather than draining a stream of messages.
That is also why a repeat run avoids duplicates: in IMAP a message carries stable traits that let the tool recognise it on the destination. POP3 offers nothing to compare.
IMAP: a1 LIST "" "*" → INBOX, Sent, Drafts, Clients/2025
POP3: LIST → 1 3402, 2 15233, 3 88110This still happens on very old hosting and on corporate gateways where IMAP is disabled by policy. The order of work is then this.
A move needs IMAP on both sides: on the source to see everything there is, on the destination to place messages into the same folders and keep their state.
If one side lacks IMAP, a migration is still possible, but it stops being an exact copy of the mailbox and becomes a transfer of the inbox. Better to know that beforehand than to discover it afterwards.
FAQ
You can fetch the inbox, but folders, flags and often dates are lost. That is not a mailbox move.
In practice provider limits set the speed, not the protocol. Whatever POP3 saves in overhead is dwarfed by the loss of structure.
By default yes. Clients usually offer to keep copies, and that setting has to be checked before the first run.
Not mandatory, but sensible: one more enabled protocol is one more way into the mailbox.
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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