To ensure that emails sent to your former domain are still delivered to your on-premises server during a transitional period after migrating your primary email to Google Workspace, you need to configure the MX (Mail Exchanger) records for the former domain to point to your on-premises email servers.

Here's why the other options are incorrect and why configuring MX records is the correct approach, based on the principles of email routing and domain management within Google Workspace:

A. Configure MX records for the former domain to point to your on-premises email servers.

MX records are DNS records that specify the mail servers responsible for accepting email messages on behalf of a domain. 1 By configuring the MX records for your former domain to point to the IP addresses or hostnames of your on-premises email servers, you are instructing the internet's DNS system that any email addressed to users on your former domain should be routed to those specific servers. This ensures that mail for the former domain bypasses Google Workspace and continues to be delivered to your existing infrastructure.  

Associate Google Workspace Administrator topics guides or documents reference: While the exact phrasing might vary across different Google Workspace support articles and documentation, the core concept of MX records and their role in email routing is fundamental to domain setup and management. The official Google Workspace Admin Help documentation on "Set up MX records for Google Workspace" (or similar titles) explicitly explains how MX records control where email for a domain is delivered. In this scenario, you are essentially managing the MX records for a domain that is not the primary Google Workspace domain to direct its mail flow.

B. Add the former domain as a secondary domain in your Google Workspace settings and verify the domain.

Adding a domain as a secondary domain within Google Workspace allows you to create separate user accounts with email addresses on that domain, all managed within your Google Workspace organization. This would mean that Google Workspace would handle the email for the former domain, which is the opposite of what you need in this scenario (you want the emails to go to your on-premises server).

Associate Google Workspace Administrator topics guides or documents reference: The Google Workspace Admin Help documentation on "Add a domain or domain alias" clearly distinguishes between secondary domains and domain aliases and their respective functionalities. Secondary domains are for managing separate sets of users, not for routing mail to external servers.

C. Adjust the TTL (Time-to-Live) for the former domain to ensure a smooth transition.

TTL is the amount of time a DNS record is cached by resolving name servers. While adjusting TTL can be important when making DNS changes (like switching MX records to Google Workspace), it doesn't directly control where email is delivered. Lowering the TTL before making MX changes to point to Google Workspace helps with a faster transition, but in this case, you are not pointing the former domain's mail to Google Workspace. Therefore, adjusting the TTL alone will not achieve the desired outcome.

Associate Google Workspace Administrator topics guides or documents reference: Information on TTL is typically found within the context of DNS management best practices in Google Workspace Admin Help, often related to domain verification or MX record changes to Google. It doesn't serve as a mechanism for routing mail to external, non-Google Workspace servers for a domain that isn't managed by Google Workspace for email.

D. Add the former domain as a domain alias for the primary domain.

Adding a domain as a domain alias means that emails sent to addresses on the alias domain will be delivered to the corresponding user accounts on your primary Google Workspace domain. This is useful when you want users to receive email at multiple domain names within your Google Workspace environment. It does not route email to an external, on-premises server.

Associate Google Workspace Administrator topics guides or documents reference: The Google Workspace Admin Help documentation on "Add a domain or domain alias" clearly explains the functionality of domain aliases. It emphasizes that email sent to a domain alias is received by the users on the primary domain, not an external system.

Therefore, the only way to ensure emails sent to your former domain are still delivered to your on-premises server is by configuring the MX records for that former domain to point to your on-premises mail server.