Thunderbird stores all emails in MBOX format, but Hotmail does not provide a direct option to upload MBOX files. Because of this limitation, many users face problems when they try to import MBOX to Hotmail from Thunderbird manually. The safest approach is to first locate the Thunderbird MBOX files on your system and then use a proper migration method. A tool like the MailConverterTools MBOX Converter Tool helps transfer Thunderbird MBOX emails into Hotmail while keeping emails, attachments, and folder structure unchanged, making the process simple even for non-technical users.
Read more: https://www.mailconvertertools.com/blog/import-mbox-to-hotmail/