Configuring Coexistence with Office 365

Zoho Mail lets you provision an environment in which a set of your organization's users exist in Zoho Mail, and another set of users can exist in Office 365. This can be achieved using email routing.

Split Delivery

When you configure a domain with Zoho Mail, you will point the MX records to Zoho's designated MX values. This will ensure that any emails that your domain receives are directed to Zoho Mail's servers. But in case you want your emails to be delivered to a different server, despite using Zoho Mail, you can configure it in the Control Panel.

Split Delivery can be set up when a set of your domain's accounts exist in Zoho Mail, and the rest of the accounts exist in Office 365. So when an account that exists in Zoho receives an email, the email will be received at the Zoho servers. In case the account does not exist in Zoho, and instead, in Office 365, the email will be routed to the destination Office 365 that you have mentioned. This way you will get the benefits of Zoho Mail for one set of users, while the rest of them do not miss out on the features of Office 365.

When you configure split delivery, the server to which your MX Records are pointed will be the primary server, and the other server will be the secondary server. For example, if Zoho Mail's MX records are added in your DNS provider, Zoho Mail will be your primary server, and the split delivery configuration has to be set up here. To check how your MX records have been configured, you can perform an MX lookup. 

Prerequisites to configure Split Delivery

1. It is recommended that you create your domain's groups or distribution lists in the server to which your MX records are pointed. If your MX records point to Zoho Mail, ensure that your groups are created in Zoho Mail.
2. While configuring SPF for the domain, the SPF record should include both Zoho Mail and Office 365 records to authorize emails from both the IPs. The SPF record that includes both Zoho Mail and Office 365 values should be mentioned as:
   
   v=spf1 include:spf.protection.outlook.com include:zohomail.com -all
   
   If you use multiple email servers for your domain, you can update the server details in the same SPF record instead of multiple entries. For detailed instructions, please refer to our help document.
3. For handling multi-source authorization, please refer to our help documentation
4. Any emails addressed to email addresses that are not configured in Zoho, will be routed to the Office 365 server. In some cases, there could spam or other emails that are sent to unknown email addresses. So if the account to which it is addressed is not configured in Office 365, the emails could be rejected or a routing failure could occur.
   To avoid this, a catch-all account can be configured in Office 365 so that these emails do not go missing.
5. It is highly recommended that Zoho's routing IP address is whitelisted in the Office 365 account. Since all the routed emails will be sent via the same IP address, whitelisting will avoid the chance of emails being identified as spam and will also avoid rate-limiting of emails.

Split delivery should be set up in Zoho only after all the above requisites are addressed, and before the MX records are pointed to Zoho servers.

Configuring Split Delivery with Office 365

There are two ways in which your organization can coexist with Office 365.

You can set up Zoho Mail as your primary server, and configure the routing to your Office 365 account, in the Zoho Mail Control Panel. In this case, the MX Records need to be pointed to Zoho Mail's servers.

Instead, if you want to use Office 365 as your primary server, you need to configure routing in Office 365, and set it up such that emails are routed to Zoho Mail. Here, the MX Records should be pointed to Office 365 servers.

Zoho Mail as the primary server

If you want Zoho Mail as the primary server, follow the below configuration instructions in Zoho Mail.

1. Login to the Zoho Mail Control Panel
2. Go to the Email Routing section
3. If you have multiple domains configured with Zoho Mail, select the domain for which you want to configure routing from the Domain to enable delivery options dropdown
4. In the Destination to route emails field, you can either enter the Office 365 specific MX record value with the least priority or the name of the domain to which you want to route these emails. 
   1. If the domain for which you are configuring routing is the same as the domain to which you'd like to route emails, enter the MX record for your Office 365 account in this field. You can find the MX record for your account from the Office 365 Domains page.
   2. If you are routing the emails to a different domain, enter the name of that domain in this field.
5. In the Email Delivery options dropdown, select Split Delivery
6. Enter a Verification Email Address. This email address should be a part of the domain for which you are configuring routing, but it should be an account that exists in Office 365. Ideally, your Office 365 account's super admin address can be used here.
7. The Change SMTP envelope is used when you want the emails to be delivered to the destination that you have specified in the settings. Sometimes, you might add a different domain name in the destination field (for example, destinationdomain.com). The domain for which you are configuring routing might be mydomain.com. If this option is enabled, the email will be sent to user@destinationdomain.com, while it is actually sent to user@mydomain.com
8. If you enable the Send bounce when the mail to the destination fails, a bounce email will be sent to the email sender if email delivery fails.
9. Turn on the Status for Advanced Delivery option.
10. The Routing for group members option has to be turned on only if you have public groups, in which some users exist in Zoho Mail and the rest of the group members exist in Office 365. In that case, if an email to the group has to be delivered to group members who exist on Office 365 as well, this option has to be turned on.

Once these configurations are done, emails will be delivered to Zoho Mail for accounts that exist here and will be routed to Office 365 for users that exist here.

Office 365 as the primary server

If you want to set up Office 365 as the primary server, follow the below instructions:

After ensuring that all prerequisites are in check, follow the steps below to configure Office 365 as the primary server for split email delivery:

1. Login to your Office 365 Exchange admin centre.
2. Select Mail Flow on the left pane and choose Connectors from the drop down.
3. In the New Connectors field, select Office 365 under Connection from, and then choose Your organization's email server under Connection to.
4. Specify a connector name of your own preference in the Name field,  and click Next.
5. Specify when you want to use the connector from the below options:
   1. Select For email messages sent to all accepted domains in your organisation, if you have added all your domains in both the providers in the Use of connectors field.
   2. Select Only when email messages are sent to these domains, if you want to route emails sent to specify domains.
6. Specify the domain/domains for which you want the routing to take place and click Add.
7. Once done, click Next.
8. In the routing field, you can either enter the Zoho Mail specific MX record value with the least priority or the name of the domain to which you want to route these emails: 
   1. If the domain for which you are configuring routing is the same as the domain to which you'd like to route emails, enter the MX record mx.zoho.com in this field.
   2. If you are routing the emails to a different domain, enter the name of that domain in this field. The MX Record for this domain will be looked up and emails will be delivered to the respective server.
9. Once you've entered the MX record, click Add.
10. In the security restrictions field, choose the issued by a trusted certificate authority option and click Next.
11. Enter the mail ID of an active user who is present in the secondary server in the Validation email field and click Validate.
12. Once the validation is successful, click Next.
13. Preview your connector configurations and click Create connector. You have now successfully created the connector for the routing to take place.
14. Navigate to the left pane and choose the Mail flow option.
15. Choose Accepted domains from the dropdown. 
16. Select the domain/domains for which you want the routing to take place and enable the internal relay option.

Once these configurations are done, emails will be delivered to Office 365 for accounts that exist there and will be routed to Zoho Mail for users that exist here.