IMAP Mail Migration

A 10-GB mailbox taking two weeks is not automatically expected, but it can happen with IMAP because the migration service, source server, internet path, and batch concurrency all constrain throughput. Split routing normally affects mail flow rather than the data copy itself, so first prove the source connection from outside the network. Run several small, comparable test batches and record elapsed time, source IMAP connections, firewall limits, and Google-side quota or throttling events. In Exchange admin center, review the IMAP migration endpoint’s concurrent-migration setting; Microsoft documents 20 as the default and permits adjustment, but raise it gradually only while the source remains healthy. Also compare a test mailbox download through a normal external IMAP client. That separates source or network limits from Exchange Online. IMAP moves mail only, so if you require richer mailbox data or substantially higher throughput, assess Microsoft’s supported Google Workspace migration approach before changing coexistence design