Migration
Migrate email accounts without losing new mail
Email migration is a moving-target problem because new messages can arrive while old mail is being copied. The safest plan keeps both services available during the transition and gives users a clear point at which their devices should use the new system.
Inventory mailboxes, aliases, forwards, groups, and senders
Do not migrate only visible inboxes. Document shared addresses, catch-all behaviour, autoresponders, distribution groups, application SMTP accounts, forwarding rules, quotas, and any systems that send mail for the domain.
Create and test the destination before MX changes
Provision accounts, authenticate through webmail or a client, configure required SPF/DKIM/DMARC changes, and verify that the destination can both send and receive test mail using a controlled path.
Copy historical mail before and after cutover
Perform an initial IMAP or provider-supported migration, change MX when ready, then run a final synchronization to capture messages delivered to the old system while DNS caches were still using it.
Update clients and keep the old service available temporarily
Give users exact incoming/outgoing settings and migration timing. Monitor both systems, bounces, authentication results, and support reports until traffic has settled and no messages remain stranded at the old provider.