IMAPSize is described as 'portable freeware Windows application that allows you to quickly and efficiently manage your IMAP mail accounts. It is not another mail client' and is an app in the Office & Productivity category. There are three alternatives to IMAPSize for Online / Web-based, Windows and iPhone. The best alternative is MailStore. Other great apps like IMAPSize are MigrationWiz (Paid) and audriga Email and Groupware migration (Paid).
MigrationWiz provides on-demand mailbox migration services through a fully automated cloud-based solution that can be accessed at anytime from anywhere to perform a seamless mailbox migration on demand.
By using the Audriga migration service you can move your email accounts and groupware accounts including emails, contacts, calender data and tasks and easy, fast and secure to your new provider. No software installation is required.