Sender Rewrite

Referring to below:

Incoming Email

1. Email Forwarding Between Tenants: You can set up email forwarding rules to route emails from one tenant to another. This can be done using mail flow rules in Exchange Online.
2. External Forwarding Services: Services like ImprovMX can forward emails between tenants. However, as you mentioned, there can be DMARC issues, so you'll need to ensure proper SPF, DKIM, and DMARC configurations to avoid email spoofing and delivery issues.

Outgoing Email

1. Sender Rewriting Scheme (SRS): SRS can help rewrite the sender's address to ensure that the email appears to come from the correct domain. This is useful for maintaining the correct "from" Domain in outgoing emails.

2. Exchange Edge Transport Server: This can be used to manage mail flow between different tenants and rewrite email addresses as needed. However, it requires additional infrastructure and management.
3. Third-Party Solutions: There are third-party email gateways and services that can handle address rewriting and ensure that outgoing emails have the correct "from" Domain.

Built-In Microsoft 365 Features

Microsoft 365 has built-in features that can help manage this scenario:

• Mail Flow Rules: You can create rules in the Exchange admin center to manage how emails are routed and rewritten.
• Hybrid Configuration: If you have an on-premises Exchange server, you can set up a hybrid configuration to manage mail flow between the on-premises environment and Microsoft 365 tenants.