Using two email providers for one domain requires an explicit routing design. Publishing both providers' MX records does not divide mailboxes by employee or reliably copy every message to both systems.
IN THIS ARTICLE
01 Design the coexistence workflow
- State the purpose: a temporary migration, a pilot or an ongoing split between teams. Define when coexistence should end if it is temporary.
- Create a recipient map showing which system owns every mailbox, alias and group. Resolve duplicate or ambiguous addresses before routing changes.
- Choose the supported primary receiving service and define how it reaches the secondary system. Confirm both providers permit the proposed route and recipient validation.
- Distinguish split delivery, which sends different recipients to different systems, from dual delivery, which delivers copies to multiple inboxes.
- For a Google-based design, review the appropriate Admin console routing workflow and its prerequisites with the administrator. Do not apply a generic rule without matching the recipient map.
- Test external inbound, replies, same-domain messages across systems, invalid recipients and filtering. Check for loops and unexpected duplicate deliveries.
- Document authentication, archive coverage, support ownership and the procedure for moving a user between systems.
02 Example: map recipients before a split-delivery pilot
For an illustrative pilot, the recipient map might look like this:
| Address | Intended mailbox owner | Validation |
|---|---|---|
[email protected] |
Primary email system | External mail arrives in Ada's primary inbox. |
[email protected] |
Secondary email system | The primary route passes Ben's mail to the secondary system. |
[email protected] |
One explicitly chosen group or mailbox | Membership and replies match the agreed team workflow. |
Test a message from Ada to Ben and a reply from Ben to Ada, as well as external delivery. If either provider treats every address as local, those same-domain messages may expose the missing route. Configure the actual route using both providers' supported instructions; this map does not prescribe MX values or synchronise mailbox state.
Common questions
Can one provider be a simple backup MX?
Only when it is configured as a compatible backup delivery service. An unrelated mailbox provider is not automatically a failover system.
Will dual delivery synchronise read status and Sent folders?
It routes messages. It does not by itself synchronise every mailbox state or action between the two applications.
Why do internal messages fail while external mail works?
A provider may treat the domain as local and never send the message to the other system. Include same-domain routing in the design and test it explicitly.