Thunderbird holds more than messages: accounts, address books, filters, calendars, add-ons, and local folders may all matter. Test a replacement with a copy of your profile, not your only live installation.
Import a small mailbox and check folder counts, attachments, flags, search, and sent-mail identity. Recreate filters and calendar subscriptions, then test offline access and a large mailbox. Verify backup and restore on a second machine. Thunderbird's profile export guide notes its ZIP export size limit; large profiles need another backup method.
Keep the original profile unchanged until the new client has completed a normal week of mail and you can locate old messages reliably.
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