I am looking for a practical way to archive an old IMAP mailbox before it becomes too difficult to manage. The account has accumulated messages over several years, including emails with large attachments and folders created for different projects. I would like to keep a complete offline copy that I can access even if the original mailbox is no longer available.

One issue I have noticed with manual methods is that they usually depend on synchronization. When the mailbox contains a large number of messages, it can be difficult to confirm whether every folder has actually been downloaded. I also do not want to manually drag thousands of emails into local folders.

For this reason, I have started looking at an imap backup tool that can process the mailbox systematically. Ideally, it should allow users to select specific folders, apply a date range, and preserve important email information. Support for attachments is also important because many of the messages contain documents that I may need later.

While comparing options, I came across the SysInfo IMAP Backup Tool . It can back up IMAP mailboxes to multiple formats, including PST, MBOX, EML, MSG, PDF, CSV, JSON, DOC, and DOCX. It also offers filters, folder mapping, duplicate removal, and batch processing, which could be useful when working with multiple accounts.
My main concern now is long-term accessibility. I do not want to create an archive that requires the original mail server or a particular email client to open it. Having different export options seems useful, but I would like to know which format is most practical for future access.

Has anyone here archived a large IMAP mailbox before migrating or deleting an account? Which method did you use, and did the resulting backup retain the original folders, attachments, dates, and email details correctly?