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OfflineIMAP vs isync: catalog facts
OfflineIMAP	isync
Free Free and open source under the GPL.	Free Free and open source under the GPL.
macOS, Linux, Command line	Windows, macOS, Linux, Command line
Two-way sync into plain Maildir, which any local mail client can read Per-folder rules and filters for what gets mirrored	Much faster than Python-based synchronisers on large mailboxes Precise control over which folders sync and in which direction
Python performance is poor on very large mailboxes Configuration is a hand-written file with many options	Configuration syntax is terse and mistakes can delete mail Command line only
Downloadable app	Downloadable app
Open source	Open source
License: GPL-2.0	License: GPL-2.0
Not stated if an account is needed	Not stated if an account is needed
Not stated if it works offline	Not stated if it works offline
Official website Full listing and sources Checked September 22, 2026	Official website Full listing and sources Checked September 22, 2026

Catalog facts only. Anything “not stated” is unconfirmed. Check full listings for details.