ODM Domain Rewrite: 550 5.7.509 Access denied NDR after users are rewritten (4379179)

Error: 550 5.7.236 Your message can't be sent because your tenant has exceeded its daily limit for sending email to external recipients from your tenant's onmicrosoft.com 

This error message is due to a limitation on the .onmicrosoft.com domain usage by the Tenants.

The changes needed to solve this error need to be performed in the involved Tenants to ensure no .onmicrosoft.com is selected as the Primary Domain for the tenants. 

The following article from Microsoft Explains the issue, the reasons of it happening and what are the actions to be performed. 

https://techcommunity.microsoft.com/blog/exchange/limiting-onmicrosoft-domain-usage-for-sending-emails/4446167