Is there a reliable IMAP email backup tool for Windows and Mac?

Yes. SysInfo IMAP Backup Tool is developed for Windows and Mac OS X and can back up the IMAP emails, attachments, folders, and metadata in a local environment. The supported file formats include PST, MBOX, PDF, EML, MSG, DOC/DOCX, JSON, and CSV; moreover, there are functions like filtering by date, removal of duplicates, folder mapping, incremental migration, and batch backup via CSV. There is information on the official website regarding the compatibility of the software with Windows and Mac computers, as well as a demo version.

If you have several IMAP accounts, the Single and Batch modes can be helpful, but in the demo version up to 100 emails per folder can be backed up.