In order to avoid relay access denied when relaying mails to 0365, a domain must be added to the “accepted”.
Accepted Domains section is accessible through: Office 365 Admin > Exchange > Mail Flow > Accepted Domains.
To add a domain, determine if that will be Authoritative or Internal Relay.
- Authoritative means that the emails being received by 0365 for this particular domain is for users with mailbox in 0365. If the recipient does not have mailbox, and NDR should notify the sender.
- Internal Relay means that the email is for users with mailbox in 0365. If no mailbox is found, 0365 will route the emails to it’s outbound connector associated with this particular domain.
Below is an illustration of SMTP connection attempt via port 25, where the sending server’s access to SMTP port is being denied by the receiving server:
Project Puchufy 