Manage the Migration to Exchange Online - Page 2
According to Microsoft, POP/IMAP migration will take at most 48 hours, for "a whole organization." In my experience, even a small, five-person organization on a Xeon-based server can take over 72 hours, with CPU utilization constantly at 99 percent after a few hours of running the tool. On top of that, some emails don't migrate due to malformed headers (according to the Migration Tool logs). Also, the emails are downloaded to the server running Microsoft Online Migration Tool before they're uploaded to Microsoft Exchange Online, so low bandwidth means slow migration. This tool comes in 64- and 32-bit versions.
On the other hand, On-premise and off-premise (Hosted) Exchange migrations can be a breeze, with emails appearing on Exchange Online's Outlook Web Access while the migration is still in progress.
In the end, you want to allow ample time for migrations, and you should try to do them in batches if at all possible.
Phase 2: Change the MX record
You should use a bigger value (than the existing MX record's value) for Mail.Global.FrontBridge.com. Let the DNS propagate for 72 hours. Next, remove the old MX record.
Also, you might need to re-sync messages that went into the old email server due to changes in the MX record. Some messages might get routed into the old email server due to different Time to Live (TTL) values in DNS server configuration.
Exchange Online Back-Out Plan
If you think that migration needs no back-out plan, then you are wrong. Here are some basic backout steps you should plan for in case things go wrong.