Office 365 Groups are glorious creations. There are, however, some instances where they don't work as you anticipate (or hope). One of those scenarios is when you are configured in hybrid coexistence with the following scenario:
- Office 365 Group Writeback is enabled (for configuring permissions, see this script)
- RequireSenderAuthenticationEnabled is set to False for an Office 365 group
- Your organization's MX record is configured to point on-premises
In this scenario, external emails sent to Office 365 groups (via your organization's MX record pointing on-premises) will be returned with an NDR with the following error text:
"You do not have permission to send to this recipient."
This happens because the RequireSenderAuthentication attribute (which maps to msExchRequireAuthToSendTo) written to the synced group object is set to the constant True inside of AAD Connect (as shown in the rule "Out to AD - Group SOAInAAD"):
Which translates to this on written-back group objects:
In order to fix this, you need to either update the rule (Edit | Disable and Make a Copy) or update the msExchRequireAuthToSendTo attribute on the synced group objects if you are keeping your MX pointed on-premises, or update the MX to point to Office 365.
If you choose to edit the rule, you'll be configuring all Office 365 groups to be open to the public. If that's not something you want, then you can make a copy of the rule and give it a higher priority, and add an attribute scoping filter. That way, you can set an attribute on Office 365 groups that you want to be open and only modify those.