Connecting two accounts in Thunderbird and dragging folders across one at a time sounds reasonable for a Google Workspace-to-IMAP move, right up until the mailbox is anything but small. Folders that take hours to sync, labels that don’t map correctly to the destination server, and a nagging uncertainty about whether every message actually made it across — that’s the realistic outcome once a mailbox has years of accumulated mail. The core issue is that Gmail labels and IMAP folders are structurally different, so a label-heavy account doesn’t translate cleanly through manual drag-and-drop. The SysInfo Email Migration Tool connects to the IMAP destination directly and lets you fetch the complete folder tree and mail counts before starting, so there’s a way to verify scope upfront instead of guessing. Folder mapping and duplicate handling run automatically during the transfer, which removes the manual checking that made the Thunderbird approach unreliable past a certain mailbox size.