Funny thing happend……
For some reason new email accounts (older then 24 hours) and email aliases was not working properly within Office365.
External contacts was reporting:
tried to deliver your message, but it was rejected by the server for the recipient domain mydomain by mydomain.mail.protection.outlook.com
550 5.4.1 [<username>@<mydomain>]: Recipient address rejected: Access denied
- Accounts was is getting propagated in an accepted time frame (24 hours)
- The affected accounts is unable to receive email from external contacts – emails are not getting delivered.
- We changed the accepted domain to Internal relay to resolve the issue.
- As a precaution we enabled Conditional Sender ID and NDR backscatter to avoid spam attacks.
Articles on Mail Flow for reference:
Exchange Online Protection Limits
How Office 365 does SPF checks for customer-to-customer mail
Manage Transport Rules
Configure your spam filter policies
Submitting spam and non-spam messages to Microsoft for analysis
Advanced spam filtering options
Spam email and Office 365 environment