Since users have existing email accounts, the next step is to determine whether you need to continue routing email to your legacy email server. One potential reason is that the legacy mail server is integrated with other corporate systems, such as a customer relationship management (CRM) system, or it enforces corporate policies about acceptable content.

Since users do not have existing accounts, you can configure Gmail for direct delivery.

If your legacy email server performs actions or applies rules for email at your domain, then all email needs to pass through the legacy server. You need to configure your system for dual delivery. You will likely want to ensure that your legacy server also handles outgoing mail by making your legacy server an outbound mail gateway.

Since all users in your domain will be using Gmail, you can configure Gmail for direct delivery. Alternatively, if you want to continue delivering mail to the users' legacy accounts for testing or rollback purposes, you can configure your system for dual delivery.

Since the users with Gmail accounts also need access to their legacy email accounts, you need to configure your system for dual delivery.

To deliver mail to a Gmail inbox or to a legacy mailbox depending on the recipient, configure your domain for split delivery.