When we kick-off the migration, we only migrate the mails that are present in the inbox at that point of time. If a mail comes in after the kick-off, they will not be migrated. To overcome this, users can use forwarding option present in their GMail settings. But, there is a way to set mail forwarding rules on the domain-level known as Dual-Delivery.
Dual-Delivery is set-up by mainly to receive emails that may come to the source accounts, while the migration is in-progress. Since you should not disturb the source account during migration and since mails that come after the migration initiation will not be migrated, we use this.
Dual-Delivery means after successful delivery of mail to the source mail address, a copy will be sent to the destination mail address. It works only if you have the same usernames at both the source & destination domains. For example, if firstname.lastname@example.org gets a mail, a copy will be sent to the email@example.com.
Note: Dual Delivery is available for inbound mail only. Outbound mail (Sent mail) is never forwarded in Dual-Delivery.
If you are not able to maintain the same usernames at both source and destination, you can create an alias for the destination account that has the same username as the source account.
Example: Source-> firstname.lastname@example.org
Destinaton-> email@example.com (alias- firstname.lastname@example.org)
Note: There might occur some unintentional forward mappings set-up due to this Dual-Delivery. For example, there is a user JohnAdams<email@example.com> in the source and John Marsh<firstname.lastname@example.org>, both of whom might be non-migrating users and are not related in anyway. But if we set up Dual-Delivery, every mail that comes to John Adams will be forwarded to John Marsh, even though they both are different persons. This is because, Dual delivery is a Domain wide setting, not a user specific one.